The Ancient Life History of the Earth / A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science

Lastly, the Lower Silurian Rocks have yielded a vast number of chambered shells, referable to animals which belong to the same great division as the Cuttle-fishes (the Cephalopoda), and of which the Pearly Nautilus is the only living representative at the present day. In this group of Cephalopods the animal
Fig. 53.—Different views of Bellerophon Argo, Trenton Limestone, Canada. (After Billings.) possesses a well-developed external shell, which is divided into chambers by shelly partitions ("septa"). The animal lives in the last-formed and largest chamber of the shell, to which it is organically connected by muscular attachments. The head is furnished with long muscular processes or "arms," and can be
Fig. 54.—Different views of Maclurea crenulata, Quebec Group, Newfoundland. (After Billings.) protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or "gills;" hence all these forms are grouped together under the name of the "Tetrabranchiate" Cephalopods (Gr. tetra, four; bragchia, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered; their "arms" are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills. For this last reason they are termed the "Dibranchiate" Cephalopods (Gr. dis, twice; bragchia, gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size. The principal Lower Silurian genus is the well-known and widely-distributed Orthoceras (fig. 55). The shell in this genus agrees with that of the existing Pearly Nautilus, in consisting of numerous chambers separated by shelly partitions (or septa), the latter being perforated by a tube which runs the whole length of the shell after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its shell. The various air-chambers, though the siphuncle passes through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it. The Orthoceras further agrees with the Pearly Nautilus in the fact that the partitions or septa separating the different air-chambers are
Fig. 55.—Fragment of Orthoceras crebriseptum, Cincinnati Group, North America, of the natural size. The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.)
Fig. 56.—[14] Restoration of Orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel") by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle. simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it. In the Nautilus, however, the shell is coiled into a flat spiral; whereas in Orthoceras the shell is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front. The chief objections to the belief that the animal of the Orthoceras was essentially like that of the Pearly Nautilus are—the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of the shell. Thus, some Orthocerata have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity. These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own shell. To some, this difficulty has appeared so great that they prefer to believe that the Orthoceras did not live in its shell at all, but that its shell was an internal skeleton similar to what we shall find to exist in many of the true Cuttle-fishes. There is something to be said in favour of this view, but it would compel us to believe in the existence in Lower Silurian times of Cuttle-fishes fully equal in size to the giant "Kraken" of fable. It need only be added in this connection that the Lower Silurian rocks have yielded the remains of many other Tetrabranchiate Cephalopods besides Orthoceras. Some of these belong to Cyrtoceras, which only differs from Orthoceras in the bow-shaped form of the shell; others belong to Phragmoceras, Lituites, &c.; and, lastly; we have true Nautili, with their spiral shells, closely resembling the existing Pearly Nautilus.

[Footnote 14: This illustration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]

Whilst all the sub-kingdoms of the Invertebrate animals are represented in the Lower Silurian rocks, no traces of Vertebrate animals have ever been discovered in these ancient deposits, unless the so-called "Conodonts" found by Pander in vast numbers in strata of this age [15] in Russia should prove to be really of this nature. These problematical bodies are of microscopic size, and have the form of minute, conical, tooth-shaped spines, with sharp edges, and hollow at the base. Their original discoverer regarded them as the horny teeth of fishes allied to the Lampreys; but Owen came to the conclusion that they probably belonged to Invertebrates. The recent investigation of a vast number of similar but slightly larger bodies, of very various forms, in the Carboniferous rocks of Ohio, has led Professor Newberry to the conclusion that these singular fossils really are, as Pander thought, the teeth of Cyclostomatous fishes. The whole of this difficult question has thus been reopened, and we may yet have to record the first advent of Vertebrate animals in the Lower Silurian.

[Footnote 15: According to Pander, the "Conodonts" are found not only in the Lower Silurian beds, but also in the "Ungulite Grit" (Upper Cambrian), as well as in the Devonian and Carboniferous deposits of Russia. Should the Conodonts prove to be truly the remains of fishes, we should thus have to transfer the first appearance of vertebrates to, at any rate, as early a period as the Upper Cambrian.]

CHAPTER X.

THE UPPER SILURIAN PERIOD.

Having now treated of the Lower Silurian period at considerable length, it will not be necessary to discuss the succeeding group of the Upper Silurian in the same detail—the more so, as with a general change of species the Upper Silurian animals belong for the most part to the same great types as those which distinguish the Lower Silurian. As compared, also, as regards the total bulk of strata concerned, the thickness of the Upper Silurian is generally very much below that of the Lower Silurian, indicating that they represent a proportionately shorter period of time. In considering the general succession of the Upper Silurian beds, we shall, as before, select Wales and America as being two regions where these deposits are typically developed.

In Wales and its borders the general succession of the Upper Silurian rocks may be taken to be as follows, in ascending order (fig. 57):—

(1) The base of the Upper Silurian series is constituted by a series of arenaceous beds, to which the name of "May Hill Sandstone" was applied by Sedgwick. These are succeeded by a series of greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes of great thickness; and these two groups of beds together form what may be termed the "May Hill Group" (Upper Llandovery of Murchison). Though not very extensively developed in Britain, this zone is one very well marked by its fossils; and it corresponds with the "Clinton Group" of North America, in which similar fossils occur. In South Wales this group is clearly unconformable to the highest member of the subjacent Lower Silurian (the Llandovery group); and there is reason to believe that a similar, though less conspicuous, physical break occurs very generally between the base of the Upper and the summit of the Lower Silurian.

(2) The Wenlock Group succeeds the May Hill group, and constitutes the middle member of the Upper Silurian. At its base it may have an irregular limestone ("Woolhope Limestone"), and its summit may be formed by a similar but thicker calcareous deposit ("Wenlock Limestone"); but the bulk of the group is made up of the argillaceous and shaly strata known as the "Wenlock Shale." In North Wales the Wenlock group is, represented by a great accumulation of flaggy and gritty strata (the "Denbighshire Flags and Grits"), and similar beds (the "Coniston Flags" and "Coniston Grits") take the same place in the north of England.

(3) The Ludlow Group is the highest member of the Upper Silurian, and consists typically of a lower arenaceous and shaly series (the "Lower Ludlow Rock") a middle calcareous member (the "Aymestry Limestone"), and an upper shaly and sandy series (the "Upper Ludlow Rock" and "Downton Sandstone"). At the summit, or close to the summit, of the Upper Ludlow, is a singular stratum only a few inches thick (varying from an inch to a foot), which contains numerous remains of crustaceans and fishes, and is well known under the name of the "bone-bed." Finally, the Upper Ludlow rock graduates invariably into a series of red sandy deposits, which, when of a flaggy character, are known locally as the "Tile-stones." These beds are probably to be regarded as the highest member of the Upper Silurian; but they are sometimes looked upon as passage-beds into the Old Red Sandstone, or as the base of this formation. It is, in fact, apparently impossible to draw any actual line of demarcation between the Upper Silurian and the overlying deposits of the Devonian or Old Red Sandstone series. Both in Britain and in America the Lower Devonian beds repose with perfect conformity upon the highest Silurian beds, and the two formations appear to pass into one another by a gradual and imperceptible transition.

The Upper Silurian strata of Britain vary from perhaps 3000 or 4000 feet in thickness up to 8000 or 10,000 feet. In North America the corresponding series, though also variable, is generally of much smaller thickness, and may be under 1000 feet. The general succession of the Upper Silurian deposits of North America is as follows:—

(1) Medina Sandstone.—This constitutes the base of the Upper Silurian, and consists of sandy strata, singularly devoid of life, and passing below in some localities into a conglomerate ("Oneida Conglomerate"), which is stated to contain pebbles derived from the older beds, and which would thus indicate an unconformity between the Upper and Lower Silurian.

(2) Clinton Group.—Above the Medina sandstone are beds of sandstone and shale, sometimes with calcareous bands, which constitute what is known as the "Clinton Group." The Medina and Clinton groups are undoubtedly the equivalent of the "May Hill Group" of Britain, as shown by the identity of their fossils.

GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND SHROPSHIRE.
Fig. 57.
(3) Niagara Group.—This group consists typically of a series of argillaceous beds ("Niagara Shale") capped by limestones ("Niagara Limestone"); and the name of the group is derived from the fact that it is over limestones of this age that the Niagara river is precipitated to form the great Falls. In places the Niagara group is wholly calcareous, and it is continued upwards into a series of marls and sandstones, with beds of salt and masses of gypsum (the "Salina Group"), or into a series of magnesian limestones ("Guelph Limestones"). The Niagara group, as a whole, corresponds unequivocally with the Wenlock group of Britain.

(4) Lower Helderberg Group.—The Upper Silurian period in North America was terminated by the deposition of a series of calcareous beds, which derive the name of "Lower Helderberg" from the Helderberg mountains, south of Albany, and which are divided into several zones, capable of recognition by their fossils, and known by local names (Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone, Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As a whole, this series may be regarded as the equivalent of the Ludlow group of Britain, though it is difficult to establish any precise parallelism. The summit of the Lower Heiderberg group is constituted by a coarse-grained sandstone (the "Oriskany Sandstone"), replete with organic remains, which have to a large extent a Silurian facies. Opinions differ as to whether this sandstone is to be regarded as the highest bed of the Upper Silurian or the base of the Devonian. We thus see that in America, as in Britain, no other line than an artificial one can be drawn between the Upper Silurian and the overlying Devonian.

As regards the life of the Upper Silurian period, we have, as before, a number of so-called "Fucoids," the true vegetable nature of which is in many instances beyond doubt. In addition to these, however, we meet for the first time, in deposits of this age, with the remains of genuine land-plants, though our knowledge of these is still too scanty to enable us to construct any detailed picture of the terrestrial vegetation of the period. Some of these remains indicate the existence of the remarkable genus Lepidodendron—a genus which played a part of great importance in the forests of the Devonian and Carboniferous periods, and which may be regarded as a gigantic and extinct type of the Club-mosses (Lycopodiaceœ). Near the summit of the Ludlow formation in Britain there have also been found beds charged with numerous small globular bodies, which Dr Hooker has shown to be the seed-vessels or "sporangia" of Club-mosses. Principal Dawson further states that he has seen in the same formation fragments of wood with the structure of the singular Devonian Conifer known as Prototaxites. Lastly, the same distinguished observer has described from the Upper Silurian of North America the remains of the singular land-plants belonging to the genus Psilophyton, which will be referred to at greater length hereafter.

The marine life of the Upper Silurian is in the main constituted by types of animals similar to those characterising the Lower Silurian, though for the most part belonging to different species. The Protozoans are represented principally by Stromatopora and Ischadites, along with a number of undoubted sponges (such as Amphispongia, Astrœospongia, Astylospongia, and Palœomanon).

Amongst the Cœlenterates, we find the old group of Graptolites now verging on extinction. Individuals still remain numerous, but the variety of generic and specific types has now become greatly reduced. All the branching and complex forms of the Arenig, the twin-Graptolites
Fig. 58.—A, Monograptus priodon, slightly enlarged. B, Fragment of the same viewed from behind. C, Fragment of the same viewed in front, showing the mouths of the cellules. D, Cross-section of the same. From the Wenlock Group (Coniston Flags of the North of England). (Original.) and Dicranograpti of the Llandeilo, and the double-celled Diplograpti and Climacograpti of the Bala group, have now disappeared. In their place we have the singular Retiolites, with its curiously-reticulated skeleton; and several species of the single-celled genus Monograptus, of which a characteristic species (M. Priodon) is here figured. If we remove from this group the plant-like Dictyonemœ, which are still present, and which survive into the Devonian, no known species of Graptolite has hitherto been detected in strata higher in geological position than the Ludlow. This, therefore, presents us with the first instance we have as yet met with of the total disappearance and extinction of a great and important series of organic forms.

The Corals are very numerously represented in the Upper Silurian rocks some of the limestones (such as the Wenlock Limestone) being often largely composed of the skeletons of these animals. Almost all the known forms of this period belong to the two great divisions of the Rugose and Tabulate corals, the former being represented by species of Zaphrentis, Omphyma, Cystiphyllum, Strombodes, Acervularia, Cyathophyllum, &c.; whilst the latter belong principally to the genera Favosites, Chœtetes, Halysites, Syringopora, Heliolites, and Plasmopora. Amongst the Rugosa, the first appearance of the great and important genus Cyathophyllum, so characteristic of the Palæozoic period, is to be noted; and amongst the Tabulata we have similarly the first appearance, in force at any rate, of the widely-spread genus Favosites—the "Honeycomb-corals." The "Chain-corals" (Halysites), figured below (fig. 59), are also very common examples of the Tabulate corals during this period, though they occur likewise in the Lower Silurian.

Amongst the Echinodermata, all those orders which have hard parts capable of ready preservation are more or less largely
Fig. 59.—a, Halysites catenularia, small variety, of the natural size; b, Fragment of a large variety of the same, of the natural size; c, Fragment of limestone with the tubes of Halysites agglomerata, of the natural size; d, Vertical section of two tubes of the same, showing the tabulæ, enlarged. Niagara Limestone (Wenlock), Canada. (Original.) represented. We have no trace of the Holothurians or Sea-cucumbers; but this is not surprising, as the record of the past is throughout almost silent as to the former existence of these soft-bodied creatures, the scattered plates and spicules in their skin offering a very uncertain chance of preservation in the fossil condition. The Sea-urchins (Echinoids) are said to be represented by examples of the old genus Palœchinus. The Star-fishes (Asteroids) and the Brittle-stars (Ophiuroids) are, comparatively speaking, largely represented; the former by species of Palasterina (fig. 60), Palœaster (fig. 60), Palœocoma (fig. 60), Petraster, Glyptaster, and Lepidaster—and the latter by species of Protaster (fig. 61), Palœodiscus, Acroura, and Eucladia. The singular Cystideans, or "Globe Crinoids," with their globular or ovate, tesselated bodies (fig. 46, A, C, D,), are also not uncommon in the Upper Silurian; and if they do not become finally extinct here, they certainly survive the close of this period by but a very brief time. By far the most important, however, of the Upper Silurian Echinodenns, are the Sea-lilies or Crinoids. The limestones of this period are often largely composed of the fragmentary columns and detached plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded
Fig. 60.—Upper Silurian Star-fishes. 1, Palasterina primœva, Lower Ludlow; 2, Paloeaster Ruthveni, Lower Ludlow; 3, Palœocoma Colvini, Lower Ludlow. (After Salter.) perhaps the most exquisitely-preserved examples of this group with which we are as yet acquainted. However varied in their forms, these beautiful organisms consist of a globular, ovate, or
Fig. 61.—A, Protaster Sedgwickii, showing the disc and bases of the arms; B, Portion of an arm, greatly enlarged. Lower Ludlow. (After Salter.) pear-shaped body (the "calyx"), supported upon a longer or shorter jointed stem (or "column"). The body is covered externally with an armour of closely-fitting calcareous plates (fig. 62), and its upper surface is protected by similar but smaller plates more loosely connected by a leathery integument. From the upper surface of the body, round its margin, springs a series of longer or shorter flexible processes, composed of innumerable calcareous joints or pieces, movably united with one another. The arms are typically five in number; but they generally subdivide at least once, sometimes twice, and they are furnished with similar but
Fig. 62.—Upper Silurian Crinoids. a, Calyx and arms of Eucalyptocrinus polydactylus, Wenlock Limestone; b, Ichthyocrinus lœvis, Niagara Limestone, America; c, Taxocrinus tuberculatus, Wenlock Limestone. (After M'Coy and Hall.) more slender lateral branches or "pinnules," thus giving rise to a crown of delicate feathery plumes. The "column" is the stem by which the animal is attached permanently to the bottom of the sea; and it is composed of numerous separate plates, so jointed together that whilst the amount of movement between any two pieces must be very limited, the entire column acquires more or less flexibility, allowing the organism as a whole to wave backwards and forwards on its stalk. Into the exquisite minutiœ of structure by which the innumerable parts entering into the composition of a single Crinoid are adapted for their proper purposes in the economy of the animal, it is impossible to enter here. No period, as before said, has yielded examples of greater beauty than the Upper Silurian, the principal genera represented being Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, Glyptocrinus, Crotalocrinus, and Edriocrinus.

The tracks and burrows of Annelides are as abundant in the Upper Silurian strata as in older deposits, and have just as commonly been regarded as plants. The most abundant forms are the cylindrical, twisted bodies (Planolites), which are so frequently found on the surfaces of sandy beds, and which have been described as the stems of sea-weeds. These fossils (fig. 63), however, can be nothing more, in most
Fig. 63.—Planolites vulgaris, the filled-up burrows of a marine worm. Upper Silurian (Clinton Group), Canada. (Original.) cases, than the filled-up burrows of marine worms resembling the living Lob-worms. There are also various remains which belong to the group of the tube-inhabiting Annelides (Tubicola). Of this nature are the tubes of Serpulites and Cornultites, and the little spiral discs of Spirorbis Lewisii.

Amongst the Articulates, we still meet only with the remains of Crustaceans. Besides the little bivalved Ostracoda—which here are occasionally found of the size of beans—and various Phyllopods of different kinds, we have an abundance of Trilobites. These last-mentioned ancient types, however, are now beginning to show signs of decadence; and though still individually numerous, there is a great diminution in the number of generic types. Many of the old genera, which flourished so abundantly in Lower Silurian seas, have now died out; and the group is represented chiefly by species of Cheirurus, Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Illœnus, Calymene, Homalonotus, and Phacops—the last of these, one of the highest and most beautiful of the groups of Trilobites, attaining here its maximum of development. In the annexed illustration (fig. 64) some of the characteristic Upper Silurian Trilobites are
Fig. 64.—Upper Silurian Trilobites. a, Cheirurus bimucronatus, Wenlock and Caradoc; b, Phacops longicaudatus, Wenlock, Britain, and America; c, Phacops Downingiœ, Wenlock and Ludlow; d, Harpes ungula, Upper Silurian, Bohemia. (After Salter and Barrande.) represented—all, however, belonging to genera which have their commencement in the Lower Silurian period. In addition to the above, the Ludlow rocks of Britain and the Lower Helderberg beds of North America have yielded the remains of certain singular Crustaceans belonging to the extinct order of the Eurypterida. Some of these wonderful forms are not remarkable for their size; but others, such as Pterygotus Anglicus (fig. 65), attain a length of six feet or more, and may fairly be considered as the giants of their class. The Eurypterids are most nearly allied to the existing King-crabs (Limuli), and have the anterior end of the body covered with a great head-shield, carrying two pairs of eyes, the one simple and the other compound. The feelers are converted into pincers, whilst the last pair of limbs have their bases covered with spiny teeth so as to act as jaws, and are flattened and widened out towards their extremities so as to officiate as swimming-paddles. The hinder extremity of the body is composed of thirteen rings, which have no legs attached to them; and the last segment of the tail is either a flattened plate or a narrow, sword-shaped spine. Fragments of the skeleton are easily recognised by the peculiar scale-like markings with
Fig. 65.—Pterygotus Anglicus, viewed from the under side, reduced in size, and restored. c c, The feelers (antennæ), terminating in nipping-claws; o o, Eyes; m m, Three pairs of jointed limbs, with pointed extremities; n n, Swimming-paddles, the bases of which are spiny and act as jaws. Upper Silurian, Lanarkshire. (After Henry Woodward.) which the surface is adorned, and which look not at all unlike the scales of a fish. The most famous locality for these great Crustaceans is Lesmahagow, in Lanarkshire, where many different species have been found. The true King-crabs (Limuli) of existing seas also appear to have been represented by at least one form (Neolimulus) in the Upper Silurian.

Coming to the Mollusca, we note the occurrence of the same great groups as in the Lower Silurian. Amongst the Sea-mosses (Polyzoa), we have the ancient Lace-corals (Fenestella and Retepora), with the nearly-allied Glauconome, and species of Ptilodictya (fig. 66); whilst many forms often referred here may probably have to be transferred to the Corals, just as some so-called Corals will ultimately be removed to the present group.

The Brachiopods continued to flourish during the Upper Silurian Period in immense numbers and under a greatly increased variety of forms. The three prominent Lower Silurian genera Orthis, Strophomena, and Leptœna are still well represented, though they have lost their former preeminence. Amongst the numerous types which have now come upon the scene for the first time, or which have now a special development, are Spirifera and Pentamerus. In the first of these (fig. 69. b, c), one of the valves of the shell (the dorsal) is furnished in its interior with a pair of great calcareous spires, which served for the support of the long and fringed fleshy processes or "arms" which were attached to the sides of the mouth.[16] In the genus Pentamerus (fig. 70) the shell is curiously subdivided in its interior by calcareous plates. The Pentameri commenced their existence at the very close of the Lower Silurian (Llandovery), and
Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped frond of Rhinopora verrucosa; 1a, Portion of the surface of the same, enlarged; 2 and 2a, Phœnopora ensiformis, of the natural size and enlarged; 3 and 3a, Helopora fragilis, of the natural size and enlarged; 4 and 4a, Ptilodictya raripora, of the natural size and enlarged. The specimens are all from the Clinton Formation (May Hill Group) of Canada. (Original.) survived to the close of the Upper Silurian; but they are specially characteristic of the May Hill and Wenlock groups, both in Britain and in other regions. One species, Pentamerus galeatus, is common to Sweden, Britain, and America. Amongst the remaining Upper Silurian Brachiopods are the extraordinary Trimerellids; the old and at the same time modern Lingulœ, Discinœ, and Craniœ; together with many species of Atrypa (fig. 68, e),
Fig. 68.—Upper Silurian Brachiopods. a a', Leptocœlia plano-convexa, Clinton Group, America; b b', Rhynchonella neglecta, Clinton Group, America; c, Rhynchonella cuneata, Niagara Group, America, and Wenlock Group, Britain; d d', Orthis elelgantula, Llandeilo to Ludlow, America and Europe; e e', Atrypa hemispherica, Clinton Group, America, and Llandovery and May Hill Groups, Britain; f f', Atrypa congesta, Clinton Group, America; g g', Orthis Davidsoni, Clinton Group, America. (After Hall, Billings, and the Author.) Leptocœlia (fig. 68, a), Rhynchonella (fig. 68, b, c), Meristella (fig. 69, a, e, f), Athyris, Retzia, Chonetes, &c.

[Footnote 16: In all the Lamp-shells the mouth is provided with two long fleshy organs, which carry delicate filaments on their sides, and which are usually coiled into a spiral. These organs are known as the "arms," and it is from their presence that the name of "Brachiopoda" is derived (Gr. brachion, arm; podes, feet). In some cases the arms are merely coiled away within the shell, without any support; but in other cases they are carried upon a more or less elaborate shelly loop, often spoken of as the "carriage-spring apparatus." In the Spirifers, and in other ancient genera, this apparatus is coiled up into a complicated spiral (fig. 67). It is these "arms," with or without
Fig. 67.—Spirifera hysterica. The right-hand figure shows the interior of the dorsal valve with the calcareous spires for the support of the arms. the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]

The higher groups of the Mollusca are also largely represented in the Upper Silurian. Apart from some singular types, such as the huge and thick-shelled Megalomi of the American Wenlock formation, the Bivalves (Lamellibranchiata) present little of
Fig. 70.—Pentamerus Knightii. Wenlock and Ludlow. The right-hand figure shows the internal partitions of the shell. special interest; for though sufficiently numerous, they are rarely well preserved, and their true affinities are often uncertain. Amongst the most characteristic genera of this period may be mentioned Cardiola (fig. 71, A and C) and Pterinea
Fig. 71.—Upper Silurian Bivalves. A, Cardiola interrupta, Wenlock and Ludlow; B, Pterinea subfalcata, Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and M'Coy.) (fig. 71, B), though the latter survives to a much later date. The Univalves (Gasteropoda) are very numerous, and a few characteristic forms are here figured (fig. 72). Of these, no genus is perhaps more characteristic than Euomphalus (fig. 72, b), with its flat discoidal shell, coiled up into an oblique spiral, and deeply hollowed out on one side; but examples of this group are both of older and of more modern date. Another very extensive genus, especially in America, is Platyceras (fig. 72, a and f), with its thin fragile shell—often hardly coiled up at all—its minute spire, and its widely-expanded, often sinuated mouth. The British Acroculiœ should probably be placed here, and the group has with reason been regarded as allied to the Violet-snails (Ianthina) of the open Atlantic. The species of Platyostoma (fig. 72, h) also belong to the same family; and the entire group is continued throughout the Devonian into the Carboniferous. Amongst other well-known Upper Silurian Gasteropods are species of the genera Holopea (fig. 72, g), Holopella (fig. 72. e),
Fig. 72.—Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helderberg, America; b, Euomphalus discors, Wenlock, Britain; c, Holopella obsoleta Ludlow, Britain; d, Platyschisma helicites, Upper Ludlow, Britain; e, Holopella gracilior, Wenlock, Britain; f, Platyceras multisinuatum, Lower Helderberg, America; g, Holopea subconica, Lower Helderberg, America; h, h', Platyostoma Niagarense, Niagara Group, America. (After Hall, M'Coy, and Salter.) Platyschisma (fig. 72, d), Cyclonema, Pleurotomaria, Murchisonia, Trochonema, &c. The oceanic
Fig. 73.—Tentaculites ornatus. Upper Silurian of Europe and North America. Univalves (Heteropods) are represented mainly by species of Bellerophon; and the Winged Snails, or Pteropods, can still boast of the gigantic Thecœ and Conulariœ, which characterise yet older deposits. The commonest genus of Pteropoda, however, is Tentaculites (fig. 73), which clearly belongs here, though it has commonly been regarded as the tube of an Annelide. The shell in this group is a conical tube, usually adorned with prominent transverse rings, and often with finer transverse or longitudinal striæ as well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces.

The last and highest group of the Mollusca—that of the Cephalopoda—is still represented only by Tetrabranchiate forms; but the abundance and variety of these is almost beyond belief. Many hundreds of different species are known, chiefly belonging to the straight Orthoceratites, but the slightly-curved Cyrtoceras is only little less common. There are also numerous forms of the genera Phragmoceras, Ascoceras, Gyroteras, Lituites, and Nautilus. Here, also, are the first-known species of the genus Goniatites—a group which attains considerable importance in later deposits, and which is to be regarded as the precursor of the Ammonites of the Secondary period.

Finally, we find ourselves for the first time called upon to consider the remains of undoubted vertebrate animals, in the
Fig. 74.—Head-shield of Pteraspis Banksii, Ludlow rocks. (After Murchison.) form of Fishes. The oldest of these remains, so far as yet known, are found in the Lower Ludlow rocks, and they consist of the bony head-shields or bucklers of certain singular armoured fishes belonging to the group of the Ganoids, represented at the present day by the Sturgeons, the Gar-pikes of North America, and a few other less familiar forms. The principal Upper Silurian genus of these is Pteraspis, and the annexed illustration (fig. 74) will give some idea of the extraordinary form of the shield covering the head in these ancient fishes. The remarkable stratum near the top of the Ludlow formation known as the "bone-bed" has also yielded the remains of shark-like fishes. Some of these, for which the name of Onchus has been proposed, are in the form of compressed, slightly-curved spines (fig. 75, A), which would appear to be of the nature of the strong defensive spines implanted in front of certain of the fins in many living fishes. Besides these, have been found fragments of prickly skin
Fig. 75.—A, Spine of Onchus tenuistriatus; B, Shagreen-scales of Thelodus. Both from the "bone-bed" of the Upper Ludlow rocks. (After Murchison.) or shagreen (Sphagodus), along with minute cushion-shaped bodies (Thelodus, fig. 75, B), which are doubtless the bony scales of some fish resembling the modern Dog-fishes. As the above mentioned remains belong to two distinct, and at the same time highly-organised, groups of the fishes, it is hardly likely that we are really presented here with the first examples of this great class. On the contrary, whether the so-called "Conodonts" should prove to be the teeth of fishes or not, we are justified in expecting that unequivocal remains of this group of animals will still be found in the Lower Silurian. It is interesting, also, to note that the first appearance of fishes—the lowest class of vertebrate animals—so far as known to us at present, does not take place until after all the great sub-kingdoms of invertebrates have been long in existence; and there is no reason for thinking that future discoveries will materially affect the relative order of succession thus indicated.

LITERATURE.

From the vast and daily-increasing mass of Silurian literature, it is impossible to do more than select a small number of works which have a classical and historical interest to the English-speaking geologist, or which embody researches on special groups of Silurian animals—anything like an enumeration of all the works and papers on this subject being wholly out of the question. Apart, therefore, from numerous and in many cases extremely important memoirs, by various well-known observers, both at home and abroad, the following are some of the more weighty works to which the student may refer in investigating the physical characters and succession of the Silurian strata and their fossil contents:—

CHAPTER XI.

THE DEVONIAN AND OLD RED SANDSTONE PERIOD.

Between the summit of the Ludlow formation and the strata which are universally admitted to belong to the Carboniferous series is a great system of deposits, to which the name of "Old Red Sandstone" was originally applied, to distinguish them from certain arenaceous strata which lie above the coal ("New Red Sandstone"). The Old Red Sandstone, properly so called, was originally described and investigated as occurring in Scotland and in South Wales and its borders; and similar strata occur in the south of Ireland. Subsequently it was discovered that sediments of a different mineral nature, and containing different organic remains, intervened between the Silurian and the Carboniferous rocks on the continent of Europe, and strata with similar palæontological characters to these were found occupying a considerable area in Devonshire. The name of "Devonian" was applied to these deposits; and this title, by common usage, has come to be regarded as synonymous with the name of "Old Red Sandstone." Lastly, a magnificent series of deposits, containing marine fossils, and undoubtedly equivalent to the true "Devonian" of Devonshire, Rhenish Prussia, Belgium, and France, is found to intervene in North America between the summit of the Silurian and the base of the Carboniferous rocks.

Much difficulty has been felt in correlating the true "Devonian Rocks" with the typical "Old Red Sandstone"—this difficulty arising from the fact that though both formations are fossiliferous, the peculiar fossils of each have only been rarely and partially found associated together. The characteristic crustaceans and many of the characteristic fishes of the Old Red are wanting in the Devonian; whilst the corals and marine shells of the latter do not occur in the former. It is impossible here to enter into any discussion as to the merits of the controversy to which this difficulty has given origin. No one, however, can doubt the importance and reality of the Devonian series as an independent system of rocks to be intercalated in point of time between the Silurian and the Carboniferous. The want of agreement, both lithologically and palæontologically, between the Devonian and the Old Red, can be explained by supposing that these two formations, though wholly or in great part contemporaneous, and therefore strict equivalents, represent deposits in two different geographical areas, laid down under different conditions. On this view, the typical Devonian rocks of Europe, Britain, and North America are the deep-sea deposits of the Devonian period, or, at any rate, are genuine marine sediments formed far from land. On the other hand, the "Old Red Sandstone" of Britain and the corresponding "Gaspé Group" of Eastern Canada represent the shallow-water shore-deposits of the same period. In fact, the former of these last-mentioned deposits contains no fossils which can be asserted positively to be marine (unless the Eurypterids be considered so); and it is even conceivable that it represents the sediments of an inland sea. Accepting this explanation in the meanwhile, we may very briefly consider the general succession of the deposits of this period in Scotland, in Devonshire, and in North America.

In Scotland the "Old Red" forms a great series of arenaceous and conglomeratic strata, attaining a thickness of many thousands of feet, and divisible into three groups. Of these, the Lower Old Red Sandstone reposes with perfect conformity upon the highest beds of the Upper Silurian, the two formations being almost inseparably united by an intermediate series of "passage-beds." In mineral nature this group consists principally of massive conglomerates, sandstones, shales, and concretionary limestones; and its fossils consist chiefly of large crustaceans belonging to the family of the Eurypterids, fishes, and plants. The Middle Old Red Sandstone consists of flagstones, bituminous shales, and conglomerates, sometimes with irregular calcareous bands; and its fossils are principally fishes and plants. It may be wholly wanting, when the Upper Old Red seems to repose unconformably upon the lower division of the series. The Upper Old Red Sandstone consists of conglomerates and grits, along with a great series of red and yellow sandstones—the fossils, as before, being fishes and remains of plants. The Upper Old Red graduates upwards conformably into the Carboniferous series.

The Devonian rocks of Devonshire are likewise divisible into a lower, middle, and upper division. The Lower Devonian or Lynton Group consists of red and purple sandstones, with marine fossils, corresponding to the "Spirifer Sandstein" of Germany, and to the arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base of the American Devonian. The Middle Devonian or Ilfracombe Group consists of sandstones and flags, with calcareous slates and crystalline limestones, containing many corals. It corresponds with the great "Eifel Limestone" of the Continent, and, in a general way, with the Corniferous Limestone and Hamilton group of North America. The Upper Devonian or Pilton Group, lastly, consists of sandstones and calcareous shales which correspond with the "Clymenia Limestone" and "Cypridina Shales" of the Continent, and with the Chemung and Portage groups of North America. It seems quite possible, also, that the so-called "Carboniferous Slates" of Ireland correspond with this group, and that the former would be more properly regarded as forming the summit of the Devonian than the base of the Carboniferous.

In no country in the world, probably, is there a finer or more complete exposition of the strata intervening between the Silurian and Carboniferous deposits than in the United States. The following are the main subdivisions of the Devonian rocks in the State of New York, where the series may be regarded as being typically developed (fig. 67):—

(1) Cauda-Galli Grit and Schoharie Grit.—Considering the "Oriskany Sandstone" as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire. The Cauda-Galli Grit is so called from the abundance of a peculiar spiral fossil (Spirophyton cauda-Galli), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed.

(2) The Corniferous or Upper Helderberg Limestone.—A series of limestones usually charged with considerable quantities of siliceous matter in the shape of hornstone or chert (Lat. cornu, horn). The thickness of this group rarely exceeds 300 feet; but it is replete with fossils, more especially with the remains of corals. The Corniferous Limestone is the equivalent of the coral-bearing limestones of the Middle Devonian of Devonshire and the great "Eifel Limestone" of Germany.

(3) The Hamilton Group—consisting of shales at the base ("Marcellus shales"); flags, shales, and impure limestones ("Hamilton beds") in the middle; and again a series of shales ("Genesee Slates") at the top. The thickness of this group varies from 200 to 1200 feet, and it is richly charged with marine fossils.

(4) The Portage Group.—A great series of shales, flags, and shaly sandstones, with few fossils.

(5) The Chemung Group.—Another great series of sandstones and shales, but with many fossils. The Portage and Chemung groups may be regarded as corresponding with the Upper Devonian of Devonshire. The Chemung beds are succeeded by a great series of red sandstones and shales—the "Catskill Group"—which pass conformably upwards into the Carboniferous, and which may perhaps be regarded as the equivalent of the great sandstones of the Upper Old Red in Scotland.

Throughout the entire series of Devonian deposits in North America no unconformability or physical break of any kind has hitherto been detected; nor is there any marked interruption to the current of life, though each subdivision of the series has its own fossils. No completely natural line can thus be indicated, dividing the Devonian in this region from the Silurian on the one hand, and the Carboniferous on the other hand. At the same time, there is the most ample evidence, both stratigraphical and palæontological, as to the complete independence of the American Devonian series as a distinct life-system between the older Silurian and the later Carboniferous. The subjoined section (fig. 76) shows diagrammatically the general succession of the Devonian rocks of North America.

As regards the life of the Devonian period, we are now acquainted with a large and abundant terrestrial flora—this being the first time that we have met with a land vegetation capable of reconstruction in any fulness. By the researches of Gœppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here. As might have been anticipated, the greater number of the vegetable remains of this period have been obtained from such shallow-water deposits as the Old Red Sandstone proper and the Gaspè series of North America, and few traces of plant-life occur in the strictly marine sediments. Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (Pecopteris, Neuropteris, Alethopteris, Sphenopteris, &c.), whilst others belong to the gigantic group of the "Tree-ferns" (Psaronius, Caulopteris, &c.) Besides these there is an abundant development of the singular extinct types of the Lepidodendroids, the Sigillarioids, and the Calamites, all of which attained their maximum in the Carboniferous. Of these, the Lepidodendra may be regarded as gigantic, tree-like Club-mosses (Lycopodiaceœ); the Calamites are equally gigantic Horse-tails (Equisetaceœ); and the Sigillarioids, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers). The Devonian rocks have GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.
Fig. 76.
also yielded traces of many other plants (such as Annularia, Asterophyllites, Cardiocarpon, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures. Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the Psilophyton (fig. 77) of Dr Dawson. These singular plants have slender branching stems, with sparse needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases. The stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate "root-stocks" or creeping stems. Upon the whole,
Fig. 77.—Restoration of Psilophyton princeps. Devonian, Canada. (After Dawson.) Principal Dawson is disposed to regard Psilophyton as a "generalised type" of plants intermediate between the Ferns and the Club-mosses. Lastly, the Devonian deposits have yielded the remains of the first actual trees with which we are as yet acquainted. About the nature of some of these (Ormoxylon and Dadoxylon) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the "discs" which are characteristic of the wood of the Pines and Firs (see fig. 2). The singular genus Prototaxites, however, which occurs in an older portion of the Devonian series than the above, is not in an absolutely unchallenged position. By Principal Dawson it is regarded as the trunk of an ancient Conifer—the most ancient known; but Mr Carruthers regards it as more probably the stem of a gigantic sea-weed. The trunks of Prototaxites (fig. 78, A) vary from one to three feet in diameter, and exhibit concentric rings of growth; but its woody fibres have not hitherto been clearly demonstrated to possess discs. Before leaving the Devonian vegetation, it may be mentioned that the hornstone or chert so abundant in the Corniferous limestone of North America has been shown to contain the remains of various microscopic plants (Diatoms and Desmids). We find also in the same siliceous material the singular spherical bodies, with radiating spines, which occur so abundantly in the chalk flints, and which are termed Xanthidia. These may be regarded as probably the spore-cases of the minute plants known as Desmidiœ.