The Devonian Protozoans have still to be fully investigated. True Sponges (such as Astrtœospongia, Sphœrospongia, &c.) are not unknown; but by far the commonest representatives of this sub-kingdom in the Devonian strata are Stromatopora and its allies. These singular organisms (fig. 79) are not only very abundant in some of the Devonian limestones—both in the Old World and the New—but they often attain very large dimensions. However much they may differ in minor details, the general structure of these bodies is that of numerous, concentrically-arranged, thin, calcareous laminæ, separated by narrow interspaces, which in turn are crossed by numerous delicate vertical pillars, giving the whole mass a cellular structure, and dividing it into innumerable minute quadrangular compartments. Many of the Devonian Stromatoporœ also exhibit on their surface the rounded openings of canals, which can hardly have served any other purpose than that of permitting the sea-water to gain ready access to every part of the organism.

No true Graptolites have ever been detected in strata of of Devonian age; and the whole of this group has become extinguished—unless we refer here the still surviving Dictyonemœ. The Cœlenterates, however, Fig. 79
Fig. 79.—a, Part of the under surface of Stromatopora tuberculata, showing the wrinkled basement membrane and the openings of water-canals, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure. Corniferous Limestone, Canada. (Original.) are represented by a vast number of Corals, of beautiful forms and very varied types. The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms—so much so as to sometimes entitle the rock to be considered as representing an ancient coral-reef. In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection. In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections. In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America. When this is the case, they often come, by the action of the weather, to stand out from the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization. As before, the principal Fig. 80
Fig. 80.—Cystiphyllum vesiculosum, showing a succession of cups produces by budding from the original coral. Of the natural size. Devonian, America and Europe. (Original.) Fig. 81
Fig. 81—Zaphrentis cornicula, of the natural size. Devonian, America. (Original.)
Fig. 82
Fig. 82—Heliophyllum exiguum, viewed from in front and behind. Of the natural size. Devonian, Canada. (Original.) representatives of the Corals are still referable to the groups of the Rugosa and Tabulata. Amongst the Rugose group we find a vast number of simple "cup-corals," generally known by the quarrymen as "horns," from their shape. Of the many forms of these, the species of Cyathophyllum, Heliophyllum (fig. 82), Zaphrentis (fig. 81), and Cystiphyllum (fig. 80), are perhaps those most abundantly represented—none of these genera, however, except Heliophyllum, being peculiar to the Devonian period. There are also numerous compound Rugose corals, such as species of Eridophyllum, Diphyphyllum, Syringopora, Phillipsastrœa, and some of the forms of Cyathophyllum and Crepidophyllum (fig. 83). Some of these compound corals attain a very large size, and form of Fig. 83
Fig. 83.—Portion of a mass of Crepidophyllum Archiaci, of the natural size. Hamilton Formation, Canada. (After Billings.) themselves regular beds, which have an analogy, at any rate, with existing coral-reefs, though there are grounds for believing that these ancient types differed from the modern reef-builders in being inhabitants of deep water. The "Tabulate Corals" are hardly less abundant in the Devonian rocks than the Rugosa; and being invariably compound, they hardly yield to the latter in the dimensions of the aggregations which they sometimes form.

The commonest, and at the same time the largest, of these are the "honeycomb corals," forming the genus Favosites (figs. 84, 85), which derive both their vernacular and their technical names from their great likeness to masses of petrified honeycomb. The most abundant species are Favosites Gothlandica and F. Hemispherica, both here figured, which form masses sometimes not less than two or three feet in diameter. Whilst Favosites has acquired a popular name by its honey-combed appearance, the resemblance of Michelinia to a fossilised wasp's nest with the comb exposed is hardly less striking, and has earned for it a similar recognition from the non-scientific Fig. 84
Fig. 84.—Portion of a mass of Favosites Gothlandica, of the natural size. Upper Silurian and Devonian of Europe and America. (Original.) Billings. Fig. 85
Fig. 85.—Fragment of Favosites hemispherica, of the natural size. Upper Silurian and Devonian of America. (After Billings.) public. In addition to these, there are numerous branching or plant-like Tabulate Corals, often of the most graceful form, which are distinctive of the Devonian in all parts of the world.

The Echinoderms of the Devonian period call for little special notice. Many of the Devonian limestones are "crinoidal;" and the Crinoids are the most abundant and widely-distributed representatives of their class in the deposits of this period.

The Cystideans, with doubtful exceptions, have not been recognised in the Devonian; and their place is taken by the allied group of the "Pentremites," which will be further spoken of as occurring in the Carboniferous rocks. On the other hand, the Star-fishes, Brittle-stars, and Sea-urchins are all continued by types more or less closely allied to those of the preceding Upper Silurian.

Of the remains of Ringed-worms (Annelides), the most numerous and the most interesting are the calcareous envelopes of some small tube-inhabiting species. No one who has visited the seaside can have failed to notice the little spiral tubes of the existing Spirorbis growing attached to shells, or covering the fronds of the commoner Sea weeds (especially Fucus serratus). These tubes are inhabited by a small Annelide, and structures of a similar character occur not uncommonly from the Upper Silurian upwards. In the Devonian rocks, Spirorbis is an extremely common fossil, growing in hundreds attached to the outer surface of corals and shells, and appearing in many specific forms (figs. 86 and 87); but almost all the known Fig. 86
Fig. 87.—a, Spirobois omphalodes, natural size and enlarged. Devonian, Europe and America; b, Spirorbis Arkonensis, of the natural size and enlarged; c, The same, with the tube twisted in the reverse direction. Devonian, America. (Original.)
Fig. 87 Fig. 88.—a b, Spirorbis laxus, enlarged, Upper Silurian, America; c, Spirorbis spinulifera, of the natural size and enlarged, Devonian, Canada. (After Hall and the Author.) examples are of small size, and are liable to escape a cursory examination.

The Crustaceans of the Devonian are principally Eurypterids and Trilobites. Some of the former attain gigantic dimensions, and the quarrymen in the Scotch Old Red give them the name of "seraphim" from their singular scale-like ornamentation. The Trilobites, though still sufficiently abundant in some localites, have undergone a yet further diminution since the close of the Upper Silurian. In both America and Europe quite a number of generic types have survived from the Silurian, but few or no new ones make their appearance during this period Fig. 88
Fig. 88.—Devonian Trilobites; a, Phacops latifrons, Devonian of Britain, the Continent of Europe, and South America; b, Homalonotus armatus, Europe; c, Phacops (Trimerocephalus) lœvis, Europe; d, Head-shield of Phacops (Portlockia) granulatus, Europe. (After Salter and Burmeister.) in either the Old World or the New. The species, however, are distinct; and the principal forms belong to the genera Phacops (fig. 88, a, c, d), Homalonotus (fig. 88, b), Proetus, and Bronteus. The species figured above under the name of Phacops latifrons (fig. 88, a), has an almost world-wide distribution, being found in the Devonian of Britain, Belgium, France, Germany, Russia, Spain, and South America; whilst its place is taken in North America by the closely-allied Phacops rana. In addition to the Trilobites, the Devonian deposits have yielded the remains of a number of the minute Ostracoda, such as Entomis ("Cypridina"), Leperditia, &c., which sometimes occur in vast numbers, as in the so-called "Cypridina Slates" of the German Devonian. There are also a few forms of Phyllopods (Estheria). Taken as a whole, the Crustacean fauna of the Devonian period presents many alliances with that of the Upper Silurian, but has only slight relationships with that of the Lower Carboniferous.

Besides Crustaceans, we meet here for the first time with the remains of air-breathing Articulates, in the shape of Insects. So far, these have only been obtained from the Devonian rocks of North America, and they indicate the existence of at least four generic types, all more or less allied to the existing May-flies (Ephemeridœ). One of these interesting primitive insects, namely, Platephemera antiqua (fig. 89), appears to have measured five inches in expanse of wing; Fig. 89
Fig. 89.—Wing of Platephemera antiqua Devonian, America. (After Dawson.) and another (Xelloneura antiquorum) has attached to its wing the remains of a "stridulating-organ" similar to that possessed by the modern Grasshoppers—the instrument, as Principal Dawson remarks, of "the first music of living things that Geology as yet reveals to us."

Amongst the Mollusca, the Devonian rocks have yielded a great number of the remains of Sea-mosses (Polyzoa). Some of these belong to the ancient type Ptilodictya, which seems to disappear here, or to the allied Clathropora (fig. 90), with its fenestrated and reticulated fronds. We meet also with the graceful and delicate stems of Ceriopora (fig. 91).

The majority of the Devonian Polyzoa belong, however, to the great and important Palæozoic group of the Lace-corals (Fenestella, figs. 92 and 94, Retepora, fig. 93, Polypora, and their allies). In all these forms there is a horny skeleton, of a fan-like or funnel-shaped form, which grew attached by its base to some foreign body. The frond consists of slightly-diverging or nearly parallel branches, which are Fig. 90
Fig. 90.—Fragment of Clathropora intertexta, of the natural size and enlarged. Devonian, Canada. (Original.) Fig. 91
Fig. 91.—Fragment of Ceriopora Hamiltonensis, of the natural size and enlarged. Devonian, Canada. (Original.) either united by delicate cross-bars, or which bend alternately from side to side, and become directly united with one another at short intervals—in either case giving origin to numerous oval or oblong perforations, which communicate to the whole Fig. 92
Fig. 92.—Fragment of Fenestella magnifica, of the natural size and enlarged. Devonian, Canada. (Original.) Fig. 93
Fig. 93.—Fragment of Retepora Phillipsi, of the natural size and enlarged. Devonian, Canada. (Original.)
Fig. 94
Fig. 94.—Fragment of Fenestella cribrosa, of the natural size and enlarged. Dovonian, Canada. (Original.) plant-like colony a characteristic netted and lace-like appearance. On one of its surfaces—sometimes the internal, sometimes the external—the frond carries a number of minute chambers or "cells," which are generally borne in rows on the branches, and of which each originally contained a minute animal.

The Brachiopods still continue to be represented in great force through all the Devonian deposits, though not occurring in the true Old Red Sandstone. Besides such old types as Orthis, Strophomena, Lingula, Athyris, and Rhynchonella, we find some entirely new ones; whilst various types which only commenced their existence in the Upper Silurian, now undergo a great expansion and development. This last is especially the case with the two families of the Spiriferidœ and the Produclidœ. The Spirifers, in particular, are especially characteristic of the Devonian, both in the Old and New Worlds—some of the most typical forms, such as Spirifera mucronata (fig. 96), having the shell "winged," or with the Fig. 95
Fig. 95.—Spirifera sculptilis. Devonian, Canada. (After Billings.) Fig. 96
Fig. 96.—Spirifera mucronata. Devonian, America. (After Billings.) lateral angles prolonged to such an extent as to have earned for them the popular name of "fossil-butterflies." The closely-allied Spirifera disjunda occurs in Britain, France, Spain, Belgium, Germany, Russia, and China. The family of the Productidœ commenced to exist in the Upper Silurian, in the genus Chonetes, and we shall hereafter find it culminating in the Carboniferous in many forms of the great genus Producta[17] itself. In the Devonian period, there is an intermediate state of things, the genus Chonetes being continued in new and varied types, and the Carboniferous Produdœ being represented by many forms of the allied group Productella. Amongst other well-known Devonian Brachiopods may be mentioned the two long-lived and persistent types Atrypa reticularis (fig. 97) and Strophomena rhomboidalis (fig. 98). The former of these commences in the Upper Silurian, but is more abundantly developed in the Devonian, having a geographical range that is nothing less than world-wide; whilst the latter commences in the Lower Silurian, and, with an almost equally cosmopolitan range, survives into the Carboniferous period.

[Footnote 17: The name of this genus is often written Productus, just as Spirifera is often given in the masculine gender as Spirifer (the name originally given to it). The masculine termination to these names is, however, grammatically incorrect, as the feminine noun cochlea (shell) is in these cases understood.]

The Bivalves (Lamellibranchiata) of the Devonian call for no special comment, the genera Pterinea and Megalodon being, perhaps, the most noticeable. The Univalves Fig. 98
Fig. 98.—Strophomena rhomboidalis. Lower Silurian, Upper Silurian, and Devonian of Europe and America. (Gasteropods), also, need not be discussed in detail, though many interesting forms of this group are known. The type most abundantly represented, especially in America, is Platyceras (fig. 99), comprising thin, wide-mouthed shells, Fig. 99
Fig. 99.—Different views of Platyceras dumosum, of the natural size. Devonian, Canada. (Original.) probably most nearly allied to the existing "Bonnet-limpets," and sometimes attaining very considerable dimensions. We may also note the continuance of the genus Euomphalus, with its discoidal spiral shell. Amongst the Heteropods, the survival of Bellerophon is to be recorded; and in the "Winged-snails," or Pteropods, we find new forms of the old genera Tentaculites and Conularia (fig. 100). The latter, with its fragile, conical, and often beautifully ornamented shell, is especially noticeable.

The remains of Cephalopoda are far from uncommon in the Fig. 100
Fig. 100.—Conularia ornata, of the natural size. Devonian, Europe. Devonian deposits, all the known forms being still Tetrabranchiate. Besides the ancient types Orthoceras and Cyrtoceras, we have now a predominance of the spirally-coiled chambered shells of Goniatites and Clymenia. In the former of these the shell is shaped like that of the Nautilus; but the partitions between the chambers ("septa") are more or less lobed, folded, or angulated, and the "siphuncle" runs along the back or convex side of the shell—these being characters which approximate Goniatites to the true Ammonites of the later rocks. In Clymenia, on the other hand, whilst the shell (fig. 101) is coiled into a flat spiral, and the partitions or septa are simple or only slightly lobed, there is still this difference, as compared with the Nautilus, that the tube of the siphuncle is placed on the inner or concave side of the shell. The Fig. 101
Fig. 101.—Clymenia Sedgwickii. Devonian, Europe. species of Clymenia are exclusively Devonian in their range; and some of the limestones of this period in Germany are so richly charged with fossils of this genus as to have received the name of "Clymenien-kalk."

The sub-kingdom of the Vertebrates is still represented by Fishes only; but these are so abundant, and belong to such varied types, that the Devonian period has been appropriately called the "Age of Fishes." Amongst the existing fishes there are three great groups which are of special geological importance, as being more or less extensively represented in past time. These groups are: (1) The Bony Fishes (Teleostei), comprising most existing fishes, in which the skeleton is more or less completely converted into bone; the tail is symmetrically lobed or divided into equal moieties; and the scales are usually thin, horny, flexible plates, which overlap one another to a greater or less extent. (2) The Ganoid Fishes (Ganoidei), comprising the modern Gar-pikes, Sturgeons, &c., in which the skeleton usually more or less completely retains its primitive soft and cartilaginous condition; the tail is generally markedly unsymmetrical, being divided into two unequal lobes; and the scales (when present) have the form of plates of bone, usually covered by a layer of shining enamel. These scales may overlap; or they may be rhomboidal plates, placed edge to edge in oblique rows; or they have the form of large-sized bony plates, which are commonly united in the region of the head to form a regular buckler. (3) The Placoid Fishes, or Elasmobranchii, comprising the Sharks, Rays, and Chimœrœ of the present day, in which the skeleton is cartilaginous; the tail is unsymmetrically lobed; and the scales have the form of detached bony plates of variable size, scattered in the integument.

It is to the two last of these groups that the Devonian fishes belong, and they are more specially referable to the Ganoids. The order of the Ganoid fishes at the present day comprises but some seven or eight genera, the species of which principally or exclusively inhabit fresh waters, and all of which are confined to the northern hemisphere. As compared, therefore, with the Bony fishes, which constitute the great majority of existing forms, the Ganoids form but an extremely small and limited group. It was far otherwise, however, in Devonian times. At this period, the bony fishes are not known to have come into existence at all, and the Ganoids held almost undisputed possession of the waters. To what extent the Devonian Ganoids were confined to fresh waters remains yet to be proved; and that many of them lived in the sea is certain. It was formerly supposed that the Old Red Sandstone of Scotland and Ireland, with its abundant fish-remains, might perhaps be a fresh-water deposit, since the habitat of its fishes is uncertain, and it contains no indubitable marine fossils. It has been now shown, however, that the marine Devonian strata of Devonshire and the continent of Europe contain some of the most characteristic of the Old Red Sandstone fishes of Scotland; whilst the undoubted marine deposit of the Corniferous limestone of North America contains numerous shark-like and Ganoid fishes, including such a characteristic Old Red genus as Coccosleus. There can be little doubt, therefore, but that the majority of the Devonian fishes were truly marine in their habits, though it is probable that many of them lived in shallow water, in the immediate neighbourhood of the shore, or in estuaries.

The Devonian Galloids belong to a number of groups; and it is Fig. 102
Fig. 102.—Fishes of the Devonian rocks of America. a, Diagram of the jaws and teeth of Dinichthys Hertzeri, viewed from the front, and greatly reduced; b, Diagram of the skull of Macropetalichthys Sullivanti, reduced in size; c, A portion of the enamelled surface of the skull of the same, magnified; d, One of the scales of Onychodus sigmoides, of the natural size; e, One of the front teeth of the lower jaw of the same, of the natural size: f, Fin-spine of Machœracanthus major, a shark-like fish, reduced in size. (After Newberry.)] only possible to notice a few of the most important forms here. The modern group of the Sturgeons is represented, more or less remotely, by a few Devonian fishes—such as Asterosteus; and the great Macropetalichthys of the Corniferous limestone of North America is believed by Newberry to belong to this group. In this fish (fig. 102, b) the skull was of large size, its outer surface being covered with a tuberculated enamel; and, as in the existing Sturgeons, the mouth seems to have been wholly destitute of teeth. Somewhat allied, also, to the Sturgeons, is a singular group of armoured fishes, which is highly characteristic of the Devonian of Britain and Europe, and less so of that of America. In these curious forms the head and front extremity of the body were protected by a buckler composed of large enamelled plates, more or less firmly united to one another; whilst the hinder end of the body was naked, or was protected with small scales. Some forms of this group—such as Pteraspis and Coccosteus—date from the Upper Silurian; but they attain their maximum in the Devonian, and none of them are known to pass upwards into the overlying Carboniferous rocks. Amongst the most characteristic forms of this group may be mentioned Cephalaspis (fig. 103) and Pterichthys (fig. 104). In the former of these the head-shield is of a Fig. 103
Fig. 103.—Cephalaspis Lyellii. Old Red Sandstone, Scotland. (After Page.) crescentic shape, having its hinder angles produced backwards into long "horns," giving it the shape of a "saddler's knife." No teeth have been discovered; but the body was covered with small ganoid scales, and there was an unsymmetrical tail-fin. In Pterichthys—which, like the preceding, was first brought to light by the labours of Hugh Miller—the whole of the head and the front part of the body were defended by a buckler of firmly-united enamelled plates, whilst the rest of the body was covered with small scales. The form of the "pectoral fins" was quite unique—these having the shape of two long, curved spines, somewhat like wings, covered by finely-tuberculated ganoid plates. All the preceding forms of this group are of small size; but few fishes, living or extinct, could rival the proportions of the great Dinichthys, referred Fig. 104
Fig. 104.—Pterichthys cornutus. Old Red Sandstone, Scotland. (After Agassiz.) to this family by Newberry. In this huge fish (fig. 102, a) the head alone is over three feet in length, and the body is supposed to have been twenty-five or thirty feet long. The head was protected by a massive cuirass of bony plates firmly articulated together, but the hinder end of the body seems to have been simply enveloped in a leathery skin. The teeth are of the most formidable description, consisting in both jaws of serrated dental plates behind, and in front of enormous conical tusks (fig. 102, a). Though immensely larger, the teeth of Dinichthys present a curious resemblance to those of the existing Mud-fishes (Lepidosiren).

In another great group of Devonian Ganoids, we meet with fishes more or less closely allied to the living Polypteri (fig. 105) of the Nile and Senegal. In this group (fig. 106) the pectoral fins consist of a central scaly lobe carrying the fin-rays on both sides, the scales being sometimes rounded and overlapping (fig. 106), or more commonly rhomboidal and placed edge to edge (fig. 105, A). Numerous forms of these "Fringe-finned" Ganoids occur in the Devonian strata, such as Holoptychius, Glyotolœmus, Osteolepis, Phaneropleuron, &c. To this group is also to be ascribed the huge Onychodus (fig. 102, d and e), with its large, rounded, overlapping scales, an inch in diameter, and its powerful pointed teeth. It is to be remembered, however, that some of these "Fringe-finned" Ganoids are probably referable to the small but singular group of the "Mud-fishes" (Dipnoi), represented at the present day by the singular Lepidosiren of South America and Africa, and the Ceratodus of the rivers of Queensland.

Leaving the Ganoid fishes, it still remains to be noticed that the Devonian deposits have yielded the remains of a number of fishes more or less closely allied to the existing Sharks, Rays, and Chimœrœ (the Elasmobranchii). The majority of the forms here alluded to are allied not to the true Sharks and Dog-fishes, but to the more peaceable "Port Jackson Fig. 105
Fig. 105.—A, Polypterus, a recent Ganoid fish; B, Osteolepis, a Devonian Ganoid; a a, Pectoral fins, showing the fin-rays arranged round a central lobe. Sharks," with their blunt teeth, adapted for crushing the shells of Molluscs. The collective name of "Cestracionts" is applied to these; and we have evidence of their past existence in the Devonian seas Fig. 106
Fig. 106.—Holoptychius nobilissimus, restored. Old Red Sandstone, Scotland. A, Scale of the same. both by their teeth, and by the defensive spines which were implanted in front of a greater or less number of the fins. These are bony spines, often variously grooved, serrated, or ornamented, with hollow bases, implanted in the integument, and capable of being erected or depressed at will. Many of these "fin-spines" have been preserved to us in the fossil condition, and the Devonian rocks have yielded examples belonging to many genera. As some of the true Sharks and Dog-fishes, some of the Ganoids, and even some Bony Fishes, possess similar defences, it is often a matter of some uncertainty to what group a given spine is to be referred. One of these spines, belonging to the genus Machœracanthus, from the Devonian rocks of America, has been figured in a previous illustration (fig. 102, f).

In conclusion, a very few words may be said as to the validity of the Devonian series as an independent system of rocks, preserving in its successive strata the record of an independent system of life. Some high authorities have been inclined to the view that the Devonian formation has in nature no actual existence, but that it is made up partly of beds which should be referred to the summit of the Upper Silurian, and partly of beds which properly belong to the base of the Carboniferous. This view seems to have been arrived at in consequence of a too exclusive study of the Devonian series of the British Isles, where the physical succession is not wholly clear, and where there is a striking discrepancy between the organic remains of those two members of the series which are known as the "Old Red Sandstone" and the "Devonian" rocks proper. This discrepancy, however, is not complete; and, as we have seen, can be readily explained on the supposition that the one group of rocks presents us with the shallow water and littoral deposits of the period, while in the other we are introduced to the deep-sea accumulations of the same period. Nor can the problem at issue be solved by an appeal to the phenomena of the British area alone, be the testimony of these what it may. As a matter of fact, there is at present no sufficient ground for believing that there is any irreconcilable discordance between the succession of rocks and of life in Britain during the period which elapsed between the deposition of the Upper Ludlow and the formation of the Carboniferous Limestone, and the order of the same phenomena during the same period in other regions. Some of the Devonian types of life, as is the case with all great formations, have descended unchanged from older types; others pass upwards unchanged to the succeeding period: but the fauna and flora of the Devonian period are, as a whole, quite distinct from those of the preceding Silurian or the succeeding Carboniferous; and they correspond to an equally distinct rock-system, which in point of time holds an intermediate position between the two great groups just mentioned. As before remarked, this conclusion may be regarded as sufficiently proved even by the phenomena of the British area; but it maybe said to be rendered a certainty by the study of the Devonian deposits of the continent of Europe—or, still more, by the investigation of the vast, for the most part uninterrupted and continuous series of sediments which commenced to be laid down in North America at the beginning of the Upper Silurian, and did not cease till, at any rate, the close of the Carboniferous.

LITERATURE.

The following list comprises the more important works and memoirs to which the student of Devonian rocks and fossils may refer:—

CHAPTER XII.

THE CARBONIFEROUS PERIOD.

Overlying the Devonian formation is the great and important series of the Carboniferous Rocks, so called because workable beds of coal are more commonly and more largely developed in this formation than in any other. Workable coal-seams, however, occur in various other formations (Jurassic, Cretaceous, Tertiary), so that coal is not an exclusively Carboniferous product; whilst even in the Coal-measures themselves the coal bears but a very small proportion to the total thickness of strata, occurring only in comparatively thin beds intercalated in a great series of sandstones, shales, and other genuine aqueous sediments.

Stratigraphically, the Carboniferous rocks usually repose conformably upon the highest Devonian beds, so that the line of demarcation between the Carboniferous and Devonian formations is principally a palæontological one, founded on the observed differences in the fossils of the two groups. On the other hand, the close of the Carboniferous period seems to have been generally, though not universally, signalised by movements of the crust of the earth, so that the succeeding Permian beds often lie unconformably upon the Carboniferous sediments.

Strata of Carboniferous age have been discovered in almost every large land-area which has been sufficiently investigated; but they are especially largely developed in Britain, in various parts of the continent of Europe, and in North America. Their general composition, however, is, comparatively speaking, so uniform, that it will suffice to take a comprehensive view of the formation without considering any one area in detail, though in each region the subdivisions of the formation are known by distinctive local names. Taking such a comprehensive view, it is found that the Carboniferous series is generally divisible into a Lower and essentially calcareous group (the "Sub-Carboniferous" or "Carboniferous Limestone"); a Middle and principally arenaceous group (the "Millstone Grit"); and an Upper group, of alternating shales and sandstones, with workable seams of coal (the "Coal-measures").

I. The Carboniferous, Sub-Carboniferous, or Mountain Limestone Series constitutes the general base of the Carboniferous system. As typically developed in Britain, the Carboniferous Limestone is essentially a calcareous formation, sometimes consisting of a mass of nearly pure limestone from 1000 to 2000 feet in thickness, or at other times of successive great beds of limestone with subordinate sandstones and shales. In the north of England the base of the series consists of pebbly conglomerates and coarse sandstones; and in Scotland generally, the group is composed of massive sandstones with a comparatively feeble development of the calcareous element. In Ireland, again, the base of the Carboniferous Limestone is usually considered to be formed by a locally-developed group of grits and shales (the "Coomhola Grits" and "Carboniferous Slate"), which attain the thickness of about 5000 feet, and contain an intermixture of Devonian with Carboniferous types of fossils. Seeing that the Devonian formation is generally conformable to the Carboniferous, we need feel no surprise at this intermixture of forms; nor does it appear to be of great moment whether these strata be referred to the former or to the latter series. Perhaps the most satisfactory course is to regard the Coomhola Grits and Carboniferous Slates as "passage-beds" between the Devonian and Carboniferous; but any view that may be taken as to the position of these beds, really leaves unaffected the integrity of the Devonian series as a distinct life-system, which, on the whole, is more closely allied to the Silurian than to the Carboniferous. In North America, lastly, the Sub-Carboniferous series is never purely calcareous, though in the interior of the continent it becomes mainly so. In other regions, however, it consists principally of shales and sandstones, with subordinate beds of limestone, and sometimes with this beds of coal or deposits of clay-ironstone.

II. The Millstone Grit.—The highest beds of the Carboniferous Limestone series are succeeded, generally with perfect conformity, by a series of arenaceous beds, usually known as the Millstone Grit. As typically developed in Britain, this group consists of hard quartzose sandstones, often so large-grained and coarse in texture as to properly constitute fine conglomerates. In other cases there are regular conglomerates, sometimes with shales, limestones, and thin beds of coal—the thickness of the whole series, when well developed, varying from 1000 to 5000 feet. In North America, the Millstone Grit rarely reaches 1000 feet in thickness; and, like its British equivalent, consists of coarse sandstones and grits, sometimes with regular conglomerates. Whilst the Carboniferous Limestone was undoubtedly deposited in a tranquil ocean of considerable depth, the coarse mechanical sediments of the Millstone Grit indicate the progressive shallowing of the Carboniferous seas, and the consequent supervention of shore-conditions.

III. The Coal-measures.—The Coal-measures properly so called rest conformably upon the Millstone Grit, and usually consist of a vast series of sandstones, shales, grits, and coals, sometimes with beds of limestone, attaining in some regions a total thickness of from 7000 to nearly 14,000 feet. Beds of workable coal are by no means unknown in some areas in the inferior group of the Sub-Carboniferous; but the general statement is true, that coal is mostly obtained from the true Coal-measures—the largest known, and at present most productive coal-fields of the world being in Great Britain, North America, and Belgium. Wherever they are found, with limited exceptions, the Coal-measures present a singular general uniformity of mineral composition. They consist, namely, of an indefinite alternation of beds of sandstone, shale, and coal, sometimes with bands of clay-ironstone or beds of limestone, repeated in no constant order, but sometimes attaining the enormous aggregate thickness of 14,000 feet, or little short of 3 miles. The beds of coal differ in number and thickness in different areas, but they seldom or never exceed one-fiftieth part of the total bulk of the formation in thickness. The characters of the coal itself, and the way in which the coal-beds were deposited, will be briefly alluded to in speaking of the vegetable life of the period. In Britain, and in the Old World generally, the Coal-measures are composed partly of genuine terrestrial deposits—such as the coal—and partly of sediments accumulated in the fresh or brackish waters of vast lagoons, estuaries, and marshes. The fossils of the Coal-measures in these regions are therefore necessarily the remains either of terrestrial plants and animals, or of such forms of life as inhabit fresh or brackish waters, the occurrence of strata with marine fossils being quite a local and occasional phenomenon. In various parts of North America, on the other hand, the Coal-measures, in addition to sandstones, shales, coal-seams, and bands of clay-ironstone, commonly include beds of limestone, charged with marine remains, and indicating marine conditions. The subjoined section (fig. 107) gives, in a generalised form, the succession of the Carboniferous strata in such a British area as the north of England, where the series is developed in a typical form.

As regards the life of the Carboniferous period, we naturally find, as has been previously noticed, great differences in different parts of the entire series, corresponding to the different mode of origin of the beds. Speaking generally, the Lower Carboniferous (or the Sub-Carboniferous) is characterised by the remains of marine animals; whilst the Upper Carboniferous (or Coal-measures) is characterised by the remains of plants and terrestrial animals. In all those cases, however, in which marine beds are found in the series of the Coal-measures, as is common in America, then we find that the fossils agree in their general characters with those of the older marine deposits of the period.

Owing to the fact that coal is simply compressed and otherwise altered vegetable matter, and that it is of the highest economic value to man, the Coal-measures have been more thoroughly explored than any other group of strata of equivalent thickness in the entire geological series. Hence we have already a very extensive acquaintance with the plants of the Carboniferous period; and our knowledge on this subject is daily undergoing increase. It is not to be supposed, however, that the remains of plants are found solely in Coal-measures; GENERALIZED SECTION OF THE CARBONIFEROUS STRATA OF THE NORTH OF ENGLAND.
Fig. 107.
Fig. 107 for though most abundant towards the summit, they are found in less numbers in all parts of the series. Wherever found, they belong to the same great types of vegetation; but, before reviewing these, a few words must be said as to the origin and mode of formation of coal.

The coal-beds, as before mentioned, occur interstratified with shales, sandstones, and sometimes limestones; and there may, within the limits of a single coal-field, be as many as 80 or 100 of such beds, placed one above the other at different levels, and varying in thickness from a few inches up to 20 or 30 feet. As a general rule, each bed of coal rests upon a bed of shale or clay, which is termed the "under-clay," and in which are found numerous roots of plants; whilst the strata immediately on the top of the coal may be shaly or sandy, but in either case are generally charged with the leaves and stems of plants, and often have upright trunks passing vertically through them. When we add to this that the coal itself is, chemically, nearly wholly composed of carbon, and that its microscopic structure shows it to be composed almost entirely of fragments of stems, leaves, bark, seeds, and vegetable débris derived from land-plants, we are readily enabled to understand how the coal was formed. The "under-clay" immediately beneath the coal-bed represents an old land-surface—sometimes, perhaps, the bottom of a swamp or marsh, covered with a luxuriant vegetation; the coal bed itself represents the slow accumulation, through long periods, of the leaves, seeds, fruits, stems, and fallen trunks of this vegetation, now hardened and compressed into a fraction of its original bulk by the pressure of the superincumbent rocks; and the strata of sand or shale above the coal-bed—the so-called "roof" of the coal—represent sediments quietly deposited as the land, after a long period of repose, commenced to sink beneath the sea. On this view, the rank and long-continued vegetation which gave rise to each coal-bed was ultimately terminated by a slow depression of the surface on which the plants grew. The land-surface then became covered by the water, and aqueous sediments were accumulated to a greater or less thickness upon the dense mass of decaying vegetation below, enveloping any trunks of trees which might still be in an erect position, and preserving between their layers the leaves and branches of plants brought down from the neighbouring land by streams, or blown into the wafer by the wind. Finally, there set in a slow movement of elevation,—the old land again reappeared above the water; a new and equally luxuriant vegetation flourished upon the new land-surface; and another coal-bed was accumulated, to be preserved ultimately in a similar fashion. Some few beds of coal may have been formed by drifted vegetable matter brought down into the ocean by rivers, and deposited directly on the bottom of the sea; but in the majority of cases the coal is undeniably the result of the slow growth and decay of plants in situ: and as the plants of the coal are not marine plants, it is necessary to adopt some such theory as the above to account for the formation of coal-seams. By this theory, as is obvious, we are compelled to suppose that the vast alluvial and marshy flats upon which the coal-plants grew were liable to constantly-recurring oscillations of level, the successive land-surfaces represented by the successive coal-beds of any coal-field being thus successively buried beneath accumulations of mud or sand. We have no need, however, to suppose that these oscillations affected large areas at the same time; and geology teaches us that local elevations and depressions of the land have been matters of constant occurrence throughout the whole of past time.

All the varieties of coal (bituminous coal, anthracite; cannel-coal, &c.) show a more or less distinct "lamination"—that is to say, they are more or less obviously composed of successive thin layers, differing slightly in colour and texture. All the varieties of coal, also, consist chemically of carbon, with varying proportions of certain gaseous constituents and a small amount of incombustible mineral or "ash." By cutting thin and transparent slices of coal, we are further enabled, by means of the microscope, to ascertain precisely not only that the carbon of the coal is derived from vegetables, but also, in many cases, what kinds of plants, and what parts of these, enter into the formation of coal. When examined in this way, all coals are found to consist more or less entirely of vegetable matter; but there is considerable difference in different coals as to the exact nature of this. By Professor Huxley it has been shown that many of the English coals consist largely of accumulations of rounded discoidal sacs or bags, which are unquestionably the seed-vessels or "spore-cases" of certain of the commoner coal-plants (such as the Lepidodendra). The best bituminous coals seem to be most largely composed of these spore-cases; whilst inferior kinds possess a progressively increasing amount of the dull carbonaceous substance which is known as "mineral charcoal," and which is undoubtedly composed of "the stems and leaves of plants reduced to little more than their carbon." On the other hand, Principal Dawson finds that the American coals only occasionally exhibit spore-cases to any extent, but consist principally of the cells, vessels, and fibres of the bark, integumentary coverings, and woody portions of the Carboniferous plants.

The number of plants already known to have existed during the Carboniferous period is so great, that nothing more can be done here than to notice briefly the typical and characteristic groups of these—such as the Ferns, the Calamites, the Lepidodendroids, the Sigillarioids, and the Conifers.

In accordance with M. Brongniart's generalisation, that the Palæozoic period is, botanically speaking, the "Age of Acrogens," we find the Carboniferous plants to be still mainly referable to the Flowerless or "Cryptogamous" division of the vegetable kingdom. The flowering or "Phanerogamous" plants, which form the bulk of our existing vegetation, are hardly known, with certainty, to have existed at all in the Carboniferous era, except as represented by trees related to the existing Pines and Fig. 108
Fig. 108.—Odontopteris Schlotheimii. Carboniferous, Europe and North America. Firs, and possibly by the Cycads or "false palms."[18] Amongst the "Cryptogams," there is no more striking or beautiful group of Carboniferous plants than the Ferns. Remains of these are found all through the Carboniferous, but in exceptional numbers in the Coal-measures, and include both herbaceous forms like the majority of existing species, and arborescent forms resembling the living Tree-ferns of New Zealand. Amongst the latter, together with some new types, are examples of the genera Psaronius and Caulopteris, both of which date from the Devonian. The simply herbaceous ferns are extremely numerous, and belong to such widely-distributed and Fig. 109
Fig. 109.—Calamites cannœformis. Carboniferous Rocks, Europe and North America. largely-represented genera as Neuropteris, Odontopteris (fig. 108), Alethopteris, Pecopteris, Sphenopteris, Hymenophyllites, &c.

[Footnote 18: Whilst the vegetation of the Coal-period was mainly a terrestrial one, aquatic plants are not unknown. Sea-weeds (such as the Spirophyton cauda-Galli) are common in some of the marine strata; whilst coal, according to the researches of the Abbé Castracane, is asserted commonly to contain the siliceous envelopes of Diatoms.]

The fossils known as Calamites (fig. 109) are very common in the Carboniferous deposits, and have given occasion to an abundance of research and speculation. They present themselves as prostrate and flattened striated stems, or as similar uncompressed stems growing in an erect position, and sometimes attaining a length of twenty feet or more. Externally, the stems are longitudinally ribbed, with transverse joints at regular intervals, these joints giving origin to a whorl or branchlets, which mayor may not give origin to similar whorls of smaller branchlets still. The stems, further, were hollow, with transverse partitions at the joints, and having neither true wood nor bark, but only a thin external fibrous shell. There can be little doubt but that the Calamites are properly regarded as colossal representatives of the little Horse-tails (Equisetaceœ) of the present day. They agree with these not only in the general details of their organisation, but also in the fact that the fruit was a species of cone, bearing "spore-cases" under scales. According to Principal Dawson, the Calamites "grew in dense brakes on the sandy and muddy flats, subject to inundation, or perhaps even in water; and they had the power of budding out from the base of the stem, so as to form clumps of plants, and also of securing their foothold by numerous cord-like roots proceeding from various heights on the lower part of the stem."

The Lepidodendroids, represented mainly by the genus Lepidodendron itself (fig. 110), were large tree-like plants, which attain their maximum in the Carboniferous period, but which appear to commence in the Upper Silurian, are well represented in the Devonian, and survive in a diminished form into the Permian. The trunks of the larger species of Lepidodendron at times reach a length of fifty feet and upwards, giving off branches in a regular bifurcating manner. The bark is marked with numerous rhombic or oval scars, arranged in quincunx order, and indicating the points where the long, needle-shaped leaves were formerly attached. The fruit consisted of cones or spikes, carried at the ends of the branches, and consisting of a central axis surrounded by overlapping scales, each of which supports a "spore-case" or seed-vessel. These cones have commonly been described under the name of Lepidostrobi. In the structure of the trunk there is nothing comparable to what is found in existing trees, there being a thick bark surrounding a zone principally composed of "scalariform" vessels, this in turn enclosing a large central pith. In their general appearance the Lepidodendra bring to mind the existing Araucarian Pines; but they are true "Cryptogams," and are to be regarded as a gigantic extinct type of the modern Club-mosses (Lycopodiaceœ). They are amongst the commonest and most characteristic of the Carboniferous Fig. 110
Fig. 110.—Lepidodendron Sternbergii, Carboniferous, Europe. The central figure represents a portion of the trunk with its branches, much reduced in size. The right-hand figure is a portion of a branch with the leaves partially attached to it; and the left-hand figure represents the end of a branch bearing a cone of fructification. plants; and the majority of the "spore-cases" so commonly found in the coal appear to have been derived from the cones of Lepidodendroids.

The so-called Sigillanoids, represented mainly by Sigillaria itself (fig. 111), were no less abundant and characteristic of the Carboniferous forests than the Lepidodendra. They commence their existence, so far as known, in the Devonian period, but they attain their maximum in the Carboniferous; and—unlike the Lepidodendroids—they are not known to occur in the Permian period. They are comparatively gigantic in size, often attaining a height of from thirty to fifty feet or more; but though abundant and well preserved, great divergence of opinion prevails as to their true affinities. The name of Sigillarioids (Lat. sigilla, little seals or images) is derived from the fact that the bark is marked with seal-like impressions or leaf-scars (fig. 111).

Externally, the trunks of Sigillaria present strong longitudinal ridges, with vertical alternating rows of oval leaf-scars indicating the points where the leaves were originally Fig. 111
Fig. 111.—Fragment of the external surface of Sigillaria Grœseri, showing the ribs and leaf-scars. The left-hand figure represents a small portion enlarged. Carboniferous, Europe. attached. The trunk was furnished with a large central pith, a thick outer bark, and an intermediate woody zone,—composed, according to Dawson, partly of the disc-bearing fibres so characteristic of Conifers; but, according to Carruthers, entirely made up of the "scalariform" vessels characteristic of Cryptogams. The size of the pith was very great, and the bark seems to have been the most durable portion of the trunk. Thus we have evidence that in many cases the stumps and "stools" of Sigillariœ, standing upright in the old Carboniferous swamps, were completely hollowed out by internal decay, till nothing but an exterior shell of bark was left. Often these hollow stumps became ultimately filled up with sediment, sometimes enclosing the remains of galley-worms, land-snails, or Amphibians, which formerly found in the cavity of the trunk a congenial home; and from the sandstone or shale now filling such trunks some of the most interesting fossils of the Coal-period have been obtained. There is little certainty as to either the leaves or fruits of Sigillaria, and there is equally little certainty as to the true botanical position of these plants. By Principal Dawson they are regarded as being probably flowering plants allied to the existing "false palms" or "Cycads," but the high authority of Mr Carruthers is to be quoted in support of the belief that they are Cryptogamic, and most nearly allied to the Club-mosses.

Leaving the botanical position of Sigillaria thus undecided, we find that it is now almost universally conceded that the fossils originally described under the name of Stigmaria are the roots of Sigillaria, the actual connection between the two having been in numerous instances demonstrated in an unmistakable manner. The Stigmariœ (fig. 112) ordinarily present themselves in the form of long, compressed or rounded Fig. 112
Fig. 112.—Stigmaria ficoides. Quarter natural size. Carboniferous. fragments, the external surface of which is covered with rounded pits or shallow tubercles, each of which has a little pit or depression in its centre. From each of these pits there proceeds, in perfect examples, a long cylindrical rootlet; but in many cases these have altogether disappeared. In their internal structure, Stigmaria exhibits a central pith surrounded by a sheath of scalariform vessels, the whole enclosed in a cellular envelope. The Stigmariœ are generally found ramifying in the "under-clay," which forms the floor of a bed of coal, and which represents the ancient soil upon which the Sigillariœ grew.

The Lepidodendroids and Sigillaroids, though the first were certainly, and the second possibly, Cryptogamic or flowerless plants, must have constituted the main mass of the forests of the Coal period; but we are not without evidence of the existence at the same time of genuine "trees," in the technical sense of this term—namely, flowering plants with large woody stems. So far as is certainly known, all the true trees of the Carboniferous formation were Conifers, allied to the existing Pines and Firs. They are recognised by the great size and concentric woody rings of their prostrate, rarely erect trunks, and by the presence of disc-bearing fibres in their wood, as demonstrated by the microscope; and the principal genera which have been recognised are Dadoxylon, Palœoxylon, Araucarioxylon, and Pinites. Their fruit is not known with absolute certainty, unless it be represented, as often conjectured, by Trigonocarpon (fig. 113). The fruits known under this name are nut-like, often of Fig. 113
Fig. 113.—Trigonocarpon ovatum. Coal-measures, Britain. (After Liudley and Hutton.) considerable size, and commonly three- or six-angled. They probably originally possessed a fleshy envelope; and if truly referable to the Conifers, they would indicate that these ancient evergreens produced berries instead of cones, and thus resembled the modern Yews rather than Pines. It seems, further, that the great group of the Cycads, which are nearly allied to the Conifers, and which attained such a striking prominence in the Secondary period, probably commenced its existence during the Coal period; but these anticipatory forms are comparatively few in number, and for the most part of somewhat dubious affinities.

CHAPTER XIII.

THE CARBONIFEROUS PERIOD—Continued.

ANIMAL LIFE OF THE CARBONIFEROUS.

We have seen that there exists a great difference as to the mode of origin of the Carboniferous sediments, some being purely marine, whilst others are terrestrial; and others, again, have been formed in inland swamps and morasses, or in brackish-water lagoons, creeks, or estuaries. A corresponding difference exists necessarily in the animal remains of these deposits, and in many regions this difference is extremely well marked and striking. The great marine limestones which characterise the lower portion of the Carboniferous series in Britain, Europe, and the eastern portion of America, and the calcareous beds which are found high up in the Carboniferous in the western States of America, may, and do, often contain the remains of drifted plants; but they are essentially characterised by marine fossils; and, moreover, they can be demonstrated by the microscope to be almost wholly composed of the remains of animals which formerly inhabited the ocean. On the other hand, the animal remains of the beds accompanying the coal are typically the remains of air-breathing, terrestrial, amphibious, or aerial animals, together with those which inhabit fresh or brackish waters. Marine fossils may be found in the Coal-measures, but they are invariably confined to special horizons in the strata, and they indicate temporary depressions of the land beneath the sea. Whilst the distinction here mentioned is one which cannot fail to strike the observer, it is convenient to consider the animal life of the Carboniferous as a whole: and it is simply necessary, in so doing, to remember that the marine fossils are in general derived from the inferior portion of the system; whilst the air-breathing, fresh-water, and brackish-water forms are almost exclusively derived from the superior portion of the same.

The Carboniferous Protozoans consist mainly of Foraminifera and Sponges. The latter are still very insufficiently known, but the former are very abundant, and belong to very varied types. Thin slices of the limestones of the period, when examined by the microscope, very commonly exhibit the shells of Foraminifera in greater or less plenty. Some limestones, indeed, are made up of little else than these minute and elegant shells, often belonging to types, such as the Textularians and Rotalians, differing little or not at all from those now in existence. This is the case, for example, with the Carboniferous Limestone of Spergen Hill in Indiana (fig. 114), which is almost wholly made up of the spiral shells of a species of Endothyra. In the same way, though to a less extent, the black Carboniferous marbles of Ireland, and the similar marbles of Yorkshire, the limestones of the west of England and of Derbyshire, and the great "Scar Limestones" of the north of England, contain great numbers of Foraminiferous shells; whilst similar organisms commonly occur in the shale-beds associated with the limestones throughout the Lower Carboniferous series. One of the most interesting of the British Carboniferous forms Fig. 114
Fig. 114.—Transparent slice of Carboniferous Limestone, from Spergen Hill, Indiana, U.S., showing numerous shells of Endothyra (Rotalia), Baiteyi slightly enlarged. (Original.) is the Saccammina of Mr Henry Brady, which is sometimes present in considerable numbers in the limestones of Northumberland, Cumberland, and the west of Scotland, and which is conspicuous for the comparatively large size of its spheroidal or pear-shaped shell (reaching from an eighth to a fifth of an inch in size). More widely distributed are the generally spindle-shaped shells of Fusulina (fig. 115), which occur in vast numbers in the Carboniferous Limestone of Russia, Armenia, the Southern Alps, and Spain, similar forms occurring in equal profusion in the higher limestones which are found in the Coal-measures of the United States, in Ohio, Illinois, Indiana, Missouri, &c. Mr Henry Brady, lastly, has shown that we have in the Nummulina Pristina of the Carboniferous Limestone of Namur a genuine Fig. 115
Fig. 115.—Fusulina cylindrica, Carboniferous Limestone, Russia. Nummulite, precursor of the great and important family of the Tertiary Nummulites.

The sub-kingdom of the Cœlenterates, so far as certainly known, is represented only by Corals;[19] but the remains of these are so abundant in many of the limestones of the Carboniferous formation as to constitute a feature little or not at all less conspicuous than that afforded by the Crinoids. As is the case in the preceding period, the Corals belong, almost exclusively, to the groups of the Rugosa and Tabulata; and there is a general and striking resemblance and relationship between the coral-fauna of the Devonian as a whole, and that of the Carboniferous. Nevertheless, there is an equally decided and striking amount of difference between these successive faunas, due to the fact that the great majority of the Carboniferous species are new; whilst some of the most characteristic Devonian genera have nearly or quite disappeared, and several new genera now make their appearance for the first time. Thus, the characteristic Devonian types Heliophyllum, Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia, Endophyllum, and Cystiphyllum, have now disappeared; and the great masses of Favosites which are such a striking feature in the Devonian limestones, are represented but by one or two degenerate and puny successors. On the other hand, we meet in the Carboniferous rocks not only with entirely new genera—such as Axophyllum, Lophophyllum, and Londsdaleia—but we have an enormous expansion of certain types which had just begun to exist in the preceding period. This is especially well seen in the Case of the genus Lithostrotion (fig. 116, b), which more than any other may be considered as the predominant Carboniferous group of Corals. All the species of Lithostrotion are compound, consisting either of bundles of loosely-approximated cylindrical stems, or of similar "coral-lites" closely aggregated together into astræiform colonies, and rendered polygonal by mutual pressure. This genus has a historical interest, as having been noticed as early as in the year 1699 by Edward Lhwyd; and it is geologically important from its wide distribution in the Carboniferous rocks of both the Old and New Worlds. Many species are known, and whole beds of limestone are often found to be composed of little else than the skeletons of these ancient corals, still standing upright as they grew. Hardly less characteristic of the Carboniferous than the above is the great group of simple "cup-corals," of which Clisiophyllum is the central type. Amongst types which commenced in the Silurian and Devonian, but which are still well represented here, may be mentioned Syringopora (fig. 116, e), with its colonies of delicate cylindrical tubes united at intervals by cross-bars; Zaphrentis (fig. 116, d), with its cup-shaped skeleton and the well-marked depression (or "fossula") on one side of the calice; Amplexus (fig. 116, c), with its cylindrical, often irregularly swollen coral and short septa; Cyathophyllum (fig. 116, a), sometimes simple, sometimes forming great masses of star-like corallites; and Chœtetes, with its branched stems, and its minute, "tabulate" tubes (fig. 116, f). The above, together with other and hardly less characteristic forms, combine to constitute a coral-fauna which is not only in itself perfectly distinctive, but which is of especial interest, from the fact that almost all the varied types of which it is composed disappeared utterly before the close of the Carboniferous Fig. 116
Fig. 116—Corals of the Carboniferous Limestone. a. Cyathophyllum paracida, showing young corallites budded forth from the disc of the old one; a', One of the corallites of the same, seen in cross-section; b, Fragment of a mass of Lithostrotion irregulare; b', One of the corallites of the same, divided transversely; c, Portion of the simple cylindrical coral of Amplexus coralloides; c', Transverse section of the same species; d, Zaphrentis vermicularis, showing the depression or "fossula" on one side of the cup; e, Fragrent of a mass of Syringopora ramulosa; f, Fragment of Cœtetes tumidus; f', Portion of the same of the same, enlarged. From the Carboniferous Limestone of Britain and Belgium. (After Thomson, De Koninck, Milne-Edwards and Haime, and the Author.) period. In the first marine sediments of a calcareous nature which succeeded to the Coal-measures (the magnesian limestones of the Permian), the great group of the Rugose corals, which flourished so largely throughout the Silurian, Devonian, and Carboniferous periods, is found to have all but disappeared, and it is never again represented save sporadically and by isolated forms.