CHAPTER VIII.

THE CAMBRIAN PERIOD.

The traces of life in the Laurentian period, as we have seen, are but scanty; but the Cambrian Rocks—so called from their occurrence in North Wales and its borders ("Cambria ")—have yielded numerous remains of animals and some dubious plants. The Cambrian deposits have thus a special interest as being the oldest rocks in which occur any number of well-preserved and unquestionable organisms. We have here the remains of the first fauna, or assemblage of animals, of which we have at present knowledge. As regards their geographical distribution, the Cambrian Rocks have been recognised in many parts of the world, but there is some question as to the precise limits of the formation, and we may consider that their most typical area is in South Wales, where they have been carefully worked out, chiefly by Dr Henry Hicks. In this region, in the neighbourhood of the promontory of St David's, the Cambrian Rocks are largely developed, resting upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and overlaid by the lowest beds of the Lower Silurian. The subjoined sketch-section (fig. 27) exhibits in a general manner the succession of strata in this locality.

From this section it will be seen that the Cambrian Rocks in Wales are divided in the first place into a lower and an upper group. The Lower Cambrian is constituted at the base by a great series of grits, sandstones, conglomerates, and slates, which are known as the "Longmynd group," from their vast development in the Longmynd Hills in Shropshire, and which attain in North Wales a thickness of 8000 feet or more. The Longmynd beds are succeeded by the so-called "Menevian group," a series of sandstones, flags, and grits, about 600 feet in thickness, and containing a considerable number of fossils. The Upper Cambrian series consists in its lower portion of nearly 5000 feet of strata, principally shaly and slaty, which are known as the "Lingula Flags," from the great abundance in them of a shell referable to the genus Lingula. These are followed by 1000 feet of dark shales and flaggy sandstones, which are known as the "Tremadoc slates," from their occurrence near Tremadoc in North Wales; and these in turn are surmounted, apparently quite conformably, by the basement beds of the Lower Silurian.

GENERALIZED SECTION OF THE CAMBRIAN ROCKS IN WALES
Fig. 27.
Fig. 27 The above may be regarded as giving a typical series of the Cambrian Rocks in a typical locality; but strata of Cambrian age are known in many other regions, of which it is only possible here to allude to a few of the most important. In Scandinavia occurs a well-developed series of Cambrian deposits, representing both the lower and upper parts of the formation. In Bohemia, the Upper Cambrian, in particular, is largely developed, and constitutes the so-called "Primordial zone" of Barrande. Lastly, in North America, whilst the Lower Cambrian is only imperfectly developed, or is represented by the Huronian, the Upper Cambrian formation has a wide extension, containing fossils similar in character to the analogous strata in Europe, and known as the "Potsdam Sandstone." The subjoined table shows the chief areas where Cambrian Rocks are developed, and their general equivalency:

Like all the older Palæozoic deposits, the Cambrian Rocks, though by no means necessarily what would be called actually "metamorphic," have been highly cleaved, and otherwise altered from their original condition. Owing partly to their indurated state, and partly to their great antiquity, they are usually found in the heart of mountainous districts, which have undergone great disturbance, and have been subjected to an enormous amount of denudation. In some cases, as in the Longmynd Hills in Shropshire, they form low rounded elevations, largely covered by pasture, and with few or no elements of sublimity. In other cases, however, they rise into bold and rugged mountains, girded by precipitous cliffs. Industrially, the Cambrian Rocks are of interest, if only for the reason that the celebrated Welsh slates of Llanberis are derived from highly-cleaved beds of this age. Taken as a whole, the Cambrian formation is essentially composed of arenaceous and muddy sediments, the latter being sometimes red, but more commonly nearly black in colour. It has often been supposed that the Cambrians are a deep-sea deposit, and that we may thus account for the few fossils contained in them; but the paucity of fossils is to a large extent imaginary, and some of the Lower Cambrian beds of the Longmynd Hills would appear to have been laid down in shallow water; as they exhibit rain-prints, sun-cracks, and ripple-marks—incontrovertible evidence of their having been a shore-deposit. The occurrence, of innumerable worm-tracks and burrows in many Cambrian strata is also a proof of shallow-water conditions; and the general absence of limestones, coupled with the coarse mechanical nature of many of the sediments of the Lower Cambrian, maybe taken as pointing in the same direction.

The life of the Cambrian, though not so rich as in the succeeding Silurian period, nevertheless consists of representatives of most of the great classes of invertebrate animals. The coarse sandy deposits of the formation, which abound more particularly towards its lower part, naturally are to a large extent barren of fossils; but the muddy sediments, when not too highly cleaved, and especially towards the summit of the group, are replete with organic remains. This is also the case, in many localities at any rate, with the finer beds of the Potsdam Sandstone in America. Limestones are known to occur in only a few areas (chiefly in America), and this may account for the apparent total absence of corals. It is, however, interesting to note that, with this exception, almost all the other leading groups of Invertebrates are known to have come into existence during the Cambrian period.

Of the land-surfaces of the Cambrian period we know nothing; and there is, therefore, nothing surprising in the fact that our acquaintance with the Cambrian vegetation is confined to some marine plants or sea-weeds, often of a very obscure and problematical nature. The "Fucoidal Sandstone" of Sweden, and the "Potsdam Sandstone" of North America, have both yielded numerous remains which have been regarded as markings left by sea-weeds or "Fucoids;" but these are highly enigmatical in their characters, and would, in many instances, seem to be rather referable to the tracks and burrows of marine worms. The first-mentioned of these formations has also yielded the curious, furrowed and striated stems which have been described as a kind of land-plant under the name of Eopkyton (fig. 28). It cannot be said, however, that the vegetable origin of these singular bodies has been satisfactorily proved. Lastly, there are found in certain green and purple beds of Lower Cambrian age at Bray Head, Wicklow, Ireland, some very remarkable fossils, which are well known under Fig. 28
Fig. 28.—Fragment of Eophyton Linneanum, a supposed land-plant. Lower Cambrian, Sweden, of the natural size. the name of Oldhamia, but the true nature of which is very doubtful. The commonest form of Oldhamia (fig. 29) consists of a thread-like stem or axis, from which spring at regular intervals bundles of short filamentous branches in a fan-like manner. In the locality where it occurs, the fronds of Oldhamia are very abundant, and are spread over the surfaces of the strata in tangled layers. That it is organic is certain, and that it is a calcareous sea-weed is probable; but it may possibly belong to the sea-mosses (Polyzoa), or to the sea-firs (Sertularians).

Amongst the lower forms of animal life (Protozoa), we find the Sponges represented by the curious bodies, composed of netted fibres, to which the name of Protospongia has been given (fig. 32, a); and the comparatively gigantic, conical, or cylindrical fossils termed Archœocyathus by Mr Billings are certainly referable either to the Foraminifera Fig. 29
Fig. 29.—A portion of Oldhamia antiqua, Lower Cambrian, Wicklow, Ireland, of the natural size. (After Salter.) or to the Sponges. The almost total absence of limestones in the formation may be regarded as a sufficient explanation of the fact that the Foraminifera are not more largely and unequivocally represented; though the existence of greensands in the Cambrian beds of Wisconsin and Tennessee may be taken as an indication that this class of animals was by no means wholly wanting. The same fact may explain the total absence of corals, so far as at present known.

The group of the Echinodermata (Sea-lilies, Sea-urchins, and their allies) is represented by a few forms, which are principally of interest as being the earliest-known examples of the class. It is also worthy of note that these precursors of a group which subsequently attains such geological importance, are referable to no less than three distinct orders—the Crinoids or Sea-lilies, represented by a species of Dendrocrinus; the Cystideans by Protocystites; and the Star-fishes by Palasterina and some other forms. Only the last of these groups, however, appears to occur in the Lower Cambrian.

The Ringed-worms (Annelida), if rightly credited with all the remains usually referred to them, appear to have swarmed in the Cambrian seas. Being soft-bodied, we do not find the actual worms themselves in the fossil condition, but we have, nevertheless, abundant traces of their existence. In some cases we find vertical burrows of greater or less depth, often expanded towards their apertures, in which the worm must have actually lived (fig. 30), as various species do at the present day. In these cases, the tube must have been rendered more or less permanent by receiving a coating of mucus, or perhaps a genuine membranous secretion, from the body of the animal; and it may be found quite empty, or occupied by a cast of sand or mud. Of this nature are the burrows which have been described under the names of Scolithus and Scolecoderma, and probably the Histioderma of the Lower Cambrian of Ireland. In other cases, as in Arenicolites (fig. 32, b), the worm seems to have inhabited a double Fig. 30
Fig. 30.—Annelide-burrows (Scolithus linearus) from the Potsdam Sandstone of Canada, of the natural size. (After Billings.) burrow, shaped like the letter U, and having two openings placed close together on the surface of the stratum. Thousands of these twin-burrows occur in some of the strata of the Longmynd, and it is supposed that the worm used one opening to the burrow as an aperture of entrance, and the other as one of exit. In other cases, again, we find simply the meandering trails caused by the worm dragging its body over the surface of the mud. Markings of this kind are commoner in the Silurian Rocks, and it is generally more or less doubtful whether they may not have been caused by other marine animals, such as shellfish, whilst some of them have certainly nothing whatever to do with the worms. Lastly, the Cambrian beds often show twining cylindrical bodies, commonly more or less matted together, and not confined to the surfaces of the strata, but passing through them. These have often been regarded as the remains of sea-weeds, but it is more probable that they represent casts of the underground burrows of worms of similar habits to the common lob-worm (Arenicola) of the present day.

The Articulate animals are numerously represented in the Cambrian deposits, but exclusively by the class of Crustaceans. Some of these are little double-shelled creatures, resembling our living water-fleas (Ostracoda). A few are larger forms, and belong to the same group as the existing brine-shrimps and fairy-shrimps (Phyllopoda). One of the most characteristic of these is the Hymenocaris vermicauda of the Lingula Flags (fig. 32, d). By far the larger number of the Cambrian Crustacea belong, however, to the remarkable and wholly extinct group of the Trilobites. These extraordinary animals must have literally swarmed in the seas of the later portion of this and the whole of the succeeding period; and they survived in greatly diminished numbers till the earlier portion of the Carboniferous period. They died out, however, wholly before the close of the Palæozoic epoch, and we have no Crustaceans at the present day which can be considered as their direct representatives. They have, however, relationships of a more or less intimate character with the existing groups of the Phyllopods, the King-crabs (Limulus), and the Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the last-mentioned order, namely, the Serolis of the coasts of Patagonia, has been regarded as the nearest living ally of the Trilobites. Be this as it may, the Trilobites possessed a skeleton which, though capable of undergoing almost endless variations, was wonderfully constant in its pattern of structure, and we may briefly describe here the chief features of this.

The upper surface of the body of a Trilobite was defended by a strong shell or "crust," partly horny and partly calcareous in its composition. This shell (fig. 31) generally exhibits a very distinct "trilobation" or division into three longitudinal lobes, one central and two lateral. It also exhibits a more important and more fundamental division into three transverse portions, which are so loosely connected with one another as very commonly to be found separate. The first and most anterior of these divisions is a shield or buckler which covers the head; the second or middle portion is composed of movable rings covering the trunk ("thorax "); and the third is a shield which covers the tailor "abdomen." The head-shield (fig. 31, e) is generally more or less semicircular in shape; and its central portion, covering the stomach of the animal, is usually strongly elevated, and generally marked by lateral furrows. A little on each side of the head are placed the eyes, which are generally crescentic in shape, and resemble the eyes of insects and many existing Crustaceans in being "compound," or made up of numerous simple eyes aggregated together. So excellent is the state of preservation of many specimens of Trilobites, that the numerous individual lenses of the eyes have been uninjured, and as many as four hundred have been counted in each eye of some forms. The eyes may be supported upon prominences, but they are never carried on movable stalks (as they are in the existing lobsters and crabs); and in some of the Cambrian Trilobites, such as the little Agnosti (fig. 31 g), the animal was blind. The lateral portions of the head-shield are usually separated from the central portion by a peculiar Fig. 31
Fig. 31.—Cambrian Trilobites: a, Paradoxides Bohemicus, reduced in size; b, Ellipsocephalus Hoffi; c, Sao hirsuta; d, Conocorypke Sultzeri (all the above, together with fig. g, are from the Upper Cambrian or "Primordial Zone" of Bohemia); e, Head-shield of Dikellocephalus Celticus, from the Lingula Flags of Wales; f, Head-shield of Conocoryphe Matthewi, from the Upper Cambrian (Acadian Group) of New Brunswick; g, Agnostus rex, Bohemia; h, Tail-shield of Dikellocephalus Minnesotensis, from the Upper Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.) line of division (the so-called "facial suture") on each side; but this is also wanting in some of the Cambrian species. The backward angles of the head-shield, also, are often prolonged into spines, which sometimes reach a great length. Following the head-shield behind, we have a portion of the body which is composed of movable segments or "body-rings," and which is technically called the "thorax," Ordinarily, this region is strongly trilobed, and each ring consists of a central convex portion, and of two flatter side-lobes. The number of body-rings in the thorax is very variable (from two to twenty-six), but is fixed for the adult forms of each group of the Trilobites. The young forms have much fewer rings than the full-grown ones; and it is curious to find that the Cambrian Trilobites very commonly have either a great many rings (as in Paradoxides, fig. 31, a), or else very few (as in Agnostus, fig. 31, g). In some instances, the body-rings do not seem to have been so constructed as to allow of much movement, but in other cases this region of the body is so flexible that the animal possessed the power of rolling itself up completely, like a hedgehog; and many individuals have been permanently preserved as fossils in this defensive condition. Finally, the body of the Trilobite was completed by a tail-shield (technically termed the "pygidium"), which varies much in size and form, and is composed of a greater or less number of rings, similar to those which form the thorax, but immovably amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have been more or less entirely destitute of hard structures, with the exception of a well-developed upper lip, in the form of a plate attached to the inferior side of the head-shield in front. There is no reason to doubt that the animal possessed legs; but these structures seem to have resembled those of many living Crustaceans in being quite soft and membranous. This, at any rate, seems to have been generally the case; though structures which have been regarded as legs have been detected on the under surface of one of the larger species of Trilobites. There is also, at present, no direct evidence that the Trilobites possessed the two pairs of jointed feelers ("antennæ") which are so characteristic of recent Crustaceans.

The Trilobites vary much in size, and the Cambrian formation presents examples of both the largest and the smallest members of the order. Some of the young forms may be little bigger than a millet-seed, and some adult examples of the smaller species (such as Agnostus) may be only a few lines in length; whilst such giants of the order as Paradoxides and Asaphus may reach a length of from one to two feet. Judging from what we actually know as to the structure of the Trilobites, and also from analogous recent forms, it would seem that these ancient Crustaceans were mud-haunting creatures, denizens of shallow seas, and affecting the soft silt of the bottom rather than the clear water above. Whenever muddy sediments are found in the Cambrian and Silurian formations, there we are tolerably sure to find Trilobites, though they are by no means absolutely wanting in limestones. They appear to have crawled out upon the sea-bottom, or burrowed in the yielding mud, with the soft under surface directed downwards; and it is probable that they really derived their nutriment from the organic matter contained in the ooze amongst which they lived. The vital organs seem to have occupied the central lobe of the skeleton, by which they were protected; and a series of delicate leaf-like paddles, which probably served as respiratory organs, would appear to have been carried on the under surface of the thorax. That they had their enemies may be regarded as certain; but we have no evidence that they were furnished with any offensive weapons, or, indeed, with any means of defence beyond their hard crust, and the power, possessed by so many of them, of rolling themselves into a ball. An additional proof of the fact that they for the most part crawled along the sea-bottom is found in the occurrence of tracks and markings of various kinds, which can hardly be ascribed to any other creatures with any show of probability. That this is the true nature of some of the markings in question cannot be doubted at all; and in other cases no explanation so probable has yet been suggested. If, however, the tracks which have been described from the Potsdam Sandstone of North America under the name of Protichnites are really due to the peregrinations of some Trilobite, they must have been produced by one of the largest examples of the order.

As already said, the Cambrian Rocks are very rich in the remains of Trilobites. In the lowest beds of the series (Longmynd Rocks), representatives of some half-dozen genera have now been detected, including the dwarf Agnostus and the giant Paradoxides. In the higher beds, the number both of genera and species is largely increased; and from the great comparative abundance of individuals, the Trilobites have every right to be considered as the most characteristic fossils of the Cambrian period,—the more so as the Cambrian species belong to peculiar types, which, for the most part, died out before the commencement of the Silurian epoch.

All the remaining Cambrian fossils which demand any notice here are members of one or other division of the great class of the Mollusca, or "Shell-fish" properly so called. In the Lower Cambrian Rocks the Lamp-shells (Brachiopoda) are the principal or sole representatives of the class, and appear chiefly in three interesting and important types—namely, Lingulella, Discina, and Obolella. Of these the last (fig. 32, i) is highly characteristic of these ancient deposits; whilst Discina is one of those remarkable persistent types which, commencing at this early period, has continued to be represented by varying forms through all the intervening geological formations up to the present day. Lingulella (fig. 32, c), again, is closely allied to the existing "Goose-bill" Lamp-shell (Lingula anatina), and thus presents us with another example of an extremely long-lived type. The Lingulellœ and their successors; the Lingulœ, are singular in possessing a shell which is of a horny texture, and contains but a small proportion of calcareous matter. In the Upper Cambrian Rocks, the Lingulellœ become much more abundant, the broad satchel-shaped species known as L. Davisii (fig. 32, e) being so abundant that one of the great divisions of the Cambrian is termed the "Lingula Flags." Here, also, we meet for the first time with examples of the genus Orthis (fig. 32, f, k, l) Fig. 32
Fig. 32.—Cambrian Fossils: a, Protospongia fenestrata, Menevian Group; b, Arenicolites didymus, Longmynd Group; c, Lingulella ferruginea, Longmynd and Menevian, enlarged; d, Hymenocaris vermicauda, Lingula Flags; e, Lingulella Davisii, Lingula Flags; f, Orthis lenticularis, Lingula Flags; g, Theca Davidii, Tremadoc Slates; h, Modiolopsis Solvensis, Tremadoc Slates; i, Obolela sagittalis, interior of valve, Menevian; j, Exterior of the same; k, Orthis Hicksii, Menevian; l, Cast of the same; m, Olenus micrurus, Lingula Flags. (Alter Salter, Hicks, and Davidson.) a characteristic Palæozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.

Of the higher groups of the Mollusca the record is as yet but scanty. In the Lower Cambrian, we have but the thin, fragile, dagger-shaped shells of the free-swimming oceanic Molluscs or "Winged-snails" (Pteropoda), of which the most characteristic is the genus Theca (fig. 32, g). In the Upper Cambrian, in addition to these, we have a few Univalves (Gasteropoda), and, thanks to the researches of Dr Hicks, quite a small assemblage of Bivalves (Lamellibranchiata), though these are mostly of no great dimensions (fig. 32, h). Of the chambered Cephalopoda (Cuttle-fishes and their allies), we have but few traces; and these wholly confined to the higher beds of the formation. We meet, however, with examples of the wonderful genus Fig. 33
Fig. 33.—Fragment of Dictyonema sociale, considerably enlarged, showing the horny branches, with their connecting cross-bars, and with a row of cells on each side. (Original.) Orthoceras, with its straight, partitioned shell, which we shall find in an immense variety of forms in the Silurian rocks. Lastly, it is worthy of note that the lowest of all the groups of the Mollusca—namely, that of the Sea-mats, Sea-mosses, and Lace-corals (Polyzoa)—is only doubtfully known to have any representatives in the Cambrian, though undergoing a large and varied development in the Silurian deposits.

An exception, however, may with much probability be made to this statement in favour of the singular genus Dictyonema (fig. 33), which is highly characteristic of the highest Cambrian beds (Tremadoc Slates). This curious fossil occurs in the form of fan-like or funnel-shaped expansions, composed of slightly-diverging horny branches, which are united in a net-like manner by numerous delicate cross-bars, and exhibit a row of little cups or cells, in which the animals were contained, on each side. Dictyonema has generally been referred to the Graptolites; but it has a much greater affinity with the plant-like Sea-firs (Sertularians) or the Sea-mosses (Polyzoa), and the balance of evidence is perhaps in favour of placing it with the latter.

LITERATURE.

The following are the more important and accessible works and memoirs which may be consulted in studying the stratigraphical and palæontological relations of the Cambrian Rocks:—

In the above list, allusion has necessarily been omitted to numerous works and memoirs on the Cambrian deposits of Sweden and Norway, Central Europe, Russia, Spain, and various parts of North America, as well as to a number of important papers on the British Cambrian strata by various well-known observers. Amongst these latter may be mentioned memoirs by Prof. Phillips, and Messrs Salter, Hicks, Belt, Plant, Homfray, Ash, Holl, &c.

CHAPTER IX.

THE LOWER SILURIAN PERIOD.

The great system of deposits to which Sir Roderick Murchison applied the name of "Silurian Rocks" reposes directly upon the highest Cambrian beds, apparently without any marked unconformity, though with a considerable change in the nature of the fossils. The name "Silurian" was originally proposed by the eminent geologist just alluded to for a great series of strata lying below the Old Red Sandstone, and occupying districts in Wales and its borders which were at one time inhabited by the "Silures," a tribe of ancient Britons. Deposits of a corresponding age are now known to be largely developed in other parts of England, in Scotland, and in Ireland, in North America, in Australia, in India, in Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and in various other regions of less note. In some regions, as in the neighbourhood of St Petersburg, the Silurian strata are found not only to have preserved their original horizontality, but also to have retained almost unaltered their primitive soft and incoherent nature. In other regions, as in Scandinavia and many parts of North America, similar strata, now consolidated into shales, sandstones, and limestones, may be found resting with a very slight inclination on still older sediments. In a great many regions, however, the Silurian deposits are found to have undergone more or less folding, crumpling, and dislocation, accompanied by induration and "cleavage" of the finer and softer sediments; whilst in some regions, as in the Highlands of Scotland, actual "metamorphism" has taken place. In consequence of the above, Silurian districts usually present the bold, rugged, and picturesque outlines which are characteristic of the older "Primitive" rocks of the earth's crust in general. In many instances, we find Silurian strata rising into mountain-chains of great grandeur and sublimity, exhibiting the utmost diversity of which rock-scenery is capable, and delighting the artist with endless changes of valley, lake, and cliff. Such districts are little suitable for agriculture, though this is often compensated for by the valuable mineral products contained in the rocks. On the other hand, when the rocks are tolerably soft and uniform in their nature, or when few disturbances of the crust of the earth have taken place, we may find Silurian areas to be covered with an abundant pasturage or to be heavily timbered.

Under the head of "Silurian Rocks," Sir Roderick Murchison included all the strata between the summit of the "Longmynd." beds and the Old Red Sandstone, and he divided these into the two great groups of the Lower Silurian and Upper Silurian. It is, however, now generally admitted that a considerable portion of the basement beds of Murchison's Silurian series must be transferred—if only upon palæontological grounds—to the Upper Cambrian, as has here been done; and much controversy has been carried on as to the proper nomenclature of the Upper Silurian and of the remaining portion of Murchison's Lower Silurian. Thus, some would confine the name "Silurian" exclusively to the Upper Silurian, and would apply the name of "Cambro-Silurian" to the Lower Silurian, or would include all beds of the latter age in the "Cambrian" series of Sedgwick. It is not necessary to enter into the merits of these conflicting views. For our present purpose, it is sufficient to recognise that there exist two great groups of rocks between the highest Cambrian beds, as here defined, and the base of the Devonian or Old Red Sandstone. These two great groups are so closely allied to one another, both physically and palæontologically, that many authorities have established a third or intermediate group (the "Middle Silurian"), by which a passage is made from one into the other. This method of procedure involves disadvantages which appear to outweigh its advantages; and the two groups in question are not only generally capable of very distinct stratigraphical separation, but at the same time exhibit, together with the alliances above spoken of, so many and such important palæontological differences, that it is best to consider them separately. We shall therefore follow this course in the present instance; and pending the final solution of the controversy as to Cambrian and Silurian nomenclature, we shall distinguish these two groups of strata as the "Lower Silurian" and the "Upper Silurian."

The Lower Silurian Rocks are known already to be developed in various regions; and though their general succession in these areas is approximately the same, each area exhibits peculiarities of its own, whilst the subdivisions of each are known by special names. All, therefore, that can be attempted here, is to select two typical areas—such as Wales and North America and to briefly consider the grouping and divisions of the Lower Silurian in each.

In Wales, the line between the Cambrian and Lower Silurian is somewhat ill-defined, and is certainly not marked by any strong unconformity. There are, however; grounds for accepting the line proposed, for palæontological reasons, by Dr Hicks, in accordance with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become the highest of the Cambrian deposits of Britain. If we take this view, the Lower Silurian rocks of Wales and adjoining districts are found to have the following general succession from below upwards (fig. 34):—

1. The Arenig Group.—This group derives its name from the Arenig mountains, where it is extensively developed. It consists of about 4000 feet of slates, shales, and flags, and is divisible into a lower, middle, and upper division, of which the former is often regarded as Cambrian under the name of "Upper Tremadoc Slates."

2. The Llandeilo Group.—The thickness of this group varies from about 4000 to as much as 10,000 feet; but in this latter case a great amount of the thickness is made up of volcanic ashes and interbedded traps. The sedimentary beds of this group are principally slates and flags, the latter occasionally with calcareous bands; and the whole series can be divided into a lower, middle, and upper Llandeilo division, of which the last is the most important. The name of "Llandeilo" is derived from the town of the same name in Wales, where strata of this age were described by Murchison.

3. The Caradoc or Bala Group.—The alternative names of this group are also of local origin, and are derived, the one from Caer Caradoc in Shropshire, the other from Bala in Wales, strata of this age occurring in both localities. The series is divided into a lower and upper group, the latter chiefly composed of shales and flags, and the former of sandstones and shales, together with the important and interesting calcareous band known as the "Bala Limestone." The thickness of the entire series varies from 4000 to as much as 12,000 feet, according as it contains more or less of interstratified igneous rocks.

4. The Llandovery Group (Lower Llandovery of Murchison).—This series, as developed near the town of Llandovery, in Caermarthenshire, consists of less than 1000 feet of conglomerates, sandstones, and shales. It is probable, however, that the little calcareous band known as the "Hirnant Limestone," together with certain pale-coloured slates which lie above the Bala Limestone, though usually referred to the Caradoc series, should in reality be regarded as belonging to the Llandovery group.

The general succession of the Lower Silurian strata of Wales and its borders, attaining a maximum thickness (along with contemporaneous igneous matter) of nearly 30,000 feet, is diagramatically represented in the annexed sketch-section (fig. 34):—

GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF WALES.
Fig. 34.
Fig. 34 In North America, both in the United States and in Canada, the Silurian rocks are very largely developed, and may be regarded as constituting an exceedingly full and typical series of the deposits of this period. The chief groups of the Silurian rocks of North America are as follows, beginning, as before, with the lowest strata, and proceeding upwards (fig. 35):—

1. Quebec Group.—This group is typically developed in the vicinity of Quebec, where it consists of about 5000 feet of strata, chiefly variously-coloured shales, together with some sandstones and a few calcareous bands. It contains a number of peculiar Graptolites, by which it can be identified without question with the Arenig group of Wales and the corresponding Skiddaw Slates of the North of England. It is also to be noted that numerous Trilobites of a distinct Cambrian facies have been obtained in the limestones of the Quebec group, near Quebec. These fossils, however, have been exclusively obtained from the limestones of the group; and as these limestones are principally calcareous breccias or conglomerates, there is room for believing that these primordial fossils are really derived, in part at any rate, from fragments of an upper Cambrian limestone. In the State of New York, the Graptolitic shales of Quebec are wanting; and the base of the Silurian is constituted by the so-called "Calciferous Sand-rock" and "Chazy Limestone."[11] The first of these is essentially and typically calcareous, and the second is a genuine limestone.

[Footnote 11: The precise relations of the Quebec shales with Graptolites (Levis Formation) to the Calciferous and Chazy beds are still obscure, though there seems little doubt but that the Quebec Shales are superior to the Calciferous Sand-rock.]

2. The Trenton Group.—This is an essentially calcareous group, the various limestones of which it is composed being known as the "Bird's-eye," "Black River," and "Trenton" Limestones, of which the last is the thickest and most important. The thickness of this group is variable, and the bands of limestone in it are often separated by beds of shale.

3. The Cincinnati Group (Hudson River Formation[12]).—This group consists essentially of a lower series of shales, often black in colour and highly charged with bituminous matter (the "Utica Slates "), and of an upper series of shales, sandstones, and limestones (the "Cincinnati" rocks proper). The exact parallelism of the Trenton and Cincinnati groups with the subdivisions of the Welsh Silurian series can hardly be stated positively. Probably no precise equivalency exists; but there can be no doubt but that the Trenton and Cincinnati groups correspond, as a whole, with the Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic section (fig. 35) gives a general idea of the succession of the Lower Silurian rocks of North America:— GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.
Fig. 35.
Fig. 35

[Footnote 12: There is some difficulty about the precise nomenclature of this group. It was originally called the "Hudson River Formation;" but this name is inappropriate, as rocks of this age hardly touch anywhere the actual Hudson River itself, the rocks so called formerly being now known to be of more ancient date. There is also some want of propriety in the name of "Cincinnati Group," since the rocks which are known under this name in the vicinity of Cincinnati itself are the representatives of the Trenton Limestone, Utica Slates, and the old Hudson River group, inseparably united in what used to be called the "Blue Limestone Series."].

Of the life of the Lower Silurian period we have record in a vast number of fossils, showing that the seas of this period were abundantly furnished with living denizens. We have, however, in the meanwhile, no knowledge of the land-surfaces of the period. We have therefore no means of speculating as to the nature of the terrestrial animals of this ancient age, nor is anything known with certainty of any land-plants which may have existed. The only relics of vegetation upon which a positive opinion can be expressed belong to the obscure group of the "Fucoids," and are supposed to be the remains of sea-weeds. Some of the fossils usually placed under this head are probably not of a vegetable Fig. 36
Fig. 36.—Licrophycus Ottawaensis a "Fucoid," from the Trenton Limestone (Lower Silurian) of Canada. (After Billings.) nature at all, but others (fig. 36) appear to be unquestionable plants. The true affinities of these, however, are extremely dubious. All that can be said is, that remains which appear to be certainly vegetable, and which are most probably due to marine plants, have been recognised nearly at the base of the Lower Silurian (Arenig), and that they are found throughout the series whenever suitable conditions recur.

The Protozoans appear to have flourished extensively in the Lower Silurian seas, though to a large extent under forms which are still little understood. We have here for the first time the appearance of Foraminifera of the ordinary type—one of the most interesting observations in this collection being that made by Ehrenberg, who showed that the Lower Silurian sandstones of the neighbourhood of St Petersburg contained casts in glauconite of Foraminiferous shells, some of which are referable to the existing genera Rotalia and Texularia. True Sponges, belonging to that section of the group in which the skeleton is calcareous, are also not unknown, one of the Fig. 37
Fig. 37.—Astylospongia prœmorsa, cut vertically so as to exhibit the canal-system in the interior. Lower Silurian, Tennessee. (After Ferdinand Rœmer.) most characteristic genera being Astylospongia (fig. 37). In this genus are included more or less globular, often lobed sponges, which are believed not to have been attached to foreign bodies. In the form here figured there is a funnel-shaped cavity at the summit; and the entire mass of the sponge is perforated, as in living examples, by a system of canals which convey the sea-water to all parts of the organism. The canals by which the sea-water gains entrance open on the exterior of the sphere, and those by which it again escapes from the sponge open into the cup-shaped depression at the summit.

The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera Stromatopora and Receptaculites, the structure of which can merely be alluded to here. The specimens of Stromatopora (fig. 38) occur as hemispherical, pear-shaped, globular, or irregular masses, often of very considerable size, and sometimes demonstrably attached to foreign bodies. In their structure these masses consist of numerous thin calcareous laminæ, usually arranged concentrically, and separated by narrow interspaces. These interspaces are generally crossed by numerous vertical calcareous pillars, giving the vertical section of the fossil a lattice-like appearance. There are also usually minute pores in the concentric laminæ, by which the successive interspaces are Fig. 38
Fig. 38.—A small and perfect specimen of Stromatopora rugosa, of the natural size, from the Trenton Limestone of Canada. (After Billings.) placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-canals of the Sponges. Upon the whole, though presenting some curious affinities to the calcareous Sponges, Stromatopora is perhaps more properly regarded as a gigantic Foraminifer. If this view be correct, it is of special interest as being probably the nearest ally of Eozoön, the general appearance of the two being strikingly similar, though their minute structure is not at all the same. Lastly, in the fossils known as Receptaculites and Ischadites we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic Foraminifera. Their structure is very complex; but fragments are easily recognised by the fact that the exterior is covered with numerous rhomboidal calcareous plates, closely fitting together, and arranged in peculiar intersecting curves, presenting very much the appearance of the engine-turned case of a watch.

Passing next to the sub-kingdom of Cœlenterate animals (Zoophytes, Corals, &c.), we find that this great group, almost or wholly absent in the Cambrian, is represented in Lower Silurian deposits by a great number of forms belonging on the one hand to the true Corals, and en the other hand to the singular family of the Graptolites. If we except certain plant-like fossils which probably belong rather to the Sertularians or the Polyzoans (e.g., Dictyonema, Dendrograptus, &c.), the family of the Graptolites may be regarded as exclusively Silurian in its distribution. Not only is this the case, but it attained its maximum development almost upon its first appearance, in the Arenig Rocks; and whilst represented by a great variety of types in the Lower Silurian; it only exists in the Upper Silurian in a much diminished form. The Graptolites (Gr. grapho, I write; lithos, stone) were so named by Linnæus, from the resemblance of some of them to written or pencilled marks upon the stone, though the great naturalist himself did not believe them to be true fossils at all. They occur as linear or leaf-like bodies, sometimes simple, sometimes compound and branched; and no doubt whatever can be entertained as to their being the skeletons of composite organisms, or colonies of semi-independent animals united together by a common fleshy trunk, similar to what is observed in the colonies of the existing Sea-firs (Sertularians). This fleshy trunk or common stem of the colony was protected by a delicate horny sheath, and it gave origin to the little flower-like "polypites," which constituted the active element of the whole assemblage. These semi-independent beings were, in turn, protected each by a little horny cup or cell, directly connected with the common sheath below, and terminating above in an opening through which the polypite could protrude its tentacled head or could again withdraw itself for safety. The entire skeleton, again, was usually, if not universally, supported by a delicate horny rod or "axis," which appears to have been hollow, and which often protrudes to a greater or less extent beyond one or both of the extremities of the actual colony.

The above gives the elementary constitution of any Graptolite, but there are considerable differences as to the manner in which these elements are arranged and combined. In some forms the common stem of the colony gives origin to but a single row of cells on one side. If the common stem is a simple, straight, or slightly-curved linear body, then we have the simplest form of Graptolite known (the genus Monograptus); and it is worthy of note that these simple types do not come into existence till comparatively late (Llandeilo), and last nearly to the very close of the Upper Silurian. In other cases, whilst there is still but a single row of cells, the colony may consist of two of these simple stems springing from a common point, as in the so-called "twin Graptolites" (Didymograptus, fig. 40). This type is Fig. 39
Fig. 39.—Dichograptus octobrachiatus, a branched, "unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig). (After Hall.) entirely confined to the earlier portion of the Lower Silurian period (Arenig and Llandeilo). In other cases, again, there may be four of such stems springing from a central point (Tetragraptus). Lastly, there are numerous complex forms (such as Dichograptus, Loganograptus, &c.) in which there are eight or more of these simple branches, all arising from a common centre (fig. 39), which is sometimes furnished with a singular horny disc. These complicated branching forms, as well as the Tetragrapti, are characteristic of the horizon of the Arenig group. Similar forms, often specifically identical, are found at this horizon in Wales, in the great series of the Skiddaw Slates of the north of England, in the Quebec group in Canada, in equivalent beds in Sweden, and in certain gold-bearing slates of the same age in Victoria in Australia.

In another great group of Graptolites (including the genera Diplograptus, Dicranograptus, Climacograptus, &c.) the common stem of the colony gives origin, over part or the whole or its length, to two rows of cells, one on each side (fig. 41). These "double-celled" Graptolites are highly characteristic of the Lower Silurian deposits; and, with an exception more apparent than real in Bohemia, they are exclusively confined to strata of Lower Silurian age, and are not known to occur in the Upper Silurian. Fig. 40
Fig. 40.—Central portion of the colony of Didymegraptus divaricatus, Upper Llandeilo, Dumfresshire. (Original.) Lastly, there is a group of Graptolites (Phyllograptus, fig. 42) in which the colony is leaf-like in form, and is composed Fig. 41
Fig. 41.—Examples of Diplograptus pristis, showing variations in the appendages at the base. Upper Llandeilo, Dumfriesshire. (Original.) Fig. 42
Fig. 42.—Group of individuals of Phyllograptus typus, from the Quebec group of Canada. (After Hall.) One of the four rows of cells is hidden on the under surface. of four rows of cells springing in a cross-like manner from the common stem. These forms are highly characteristic of the Arenig group.

The Graptolites are usually found in dark-coloured, often black shales, which sometimes contain so much carbon as to become "anthracitic." They may be simply carbonaceous; but they are more commonly converted into iron-pyrites, when they glitter with the brilliant lustre of silver as they lie scattered on the surface of the rock, fully deserving in their metallic tracery the name of "written stones." They constitute one of the most important groups of Silurian fossils, and are of the greatest value in determining the precise stratigraphical position of the beds in which they occur. They present, however, special difficulties in their study; and it is still a moot point as to their precise position in the zoological scale. The balance of evidence is in favour of regarding them as an ancient and peculiar group of the Sea-firs (Hydroid Zoophytes), but some regard them as belonging rather to the Sea-mosses (Polyzoa). Under any circumstances, they cannot be directly compared either with the ordinary Sea-firs or the ordinary Sea-mosses; for these two groups consist of fixed organisms, whereas the Graptolites were certainly free-floating creatures, living at large in the open sea. The only Hydroid Zoophytes or Polyzoans which have a similar free mode of existence, have either no skeleton at all, or have hard structures quite unlike the horny sheaths of the Graptolites.

The second great group of Cœlenterate animals (Actinozoa) is represented in the Lower Silurian rocks by numerous Corals. These, for obvious reasons, are much more abundant in regions where the Lower Silurian series is largely calcareous (as in North America) than in districts like Wales, where limestones are very feebly developed. The Lower Silurian Corals, though the first of their class, and presenting certain peculiarities, may be regarded as essentially similar in nature to existing Corals. These, as is well known, are the calcareous skeletons of animals—the so-called "Coral-Zoophytes"—closely allied to the common Sea-anemones in structure and habit. A simple coral (fig. 43) consists of a calcareous cup embedded in the soft tissues of the flower-like polype, and having at its summit a more or less deep depression (the "calice") in which the digestive organs are contained. The space within the coral is divided into compartments by numerous vertical calcareous plates (the "septa"), which spring from the inside of the wall of the cup, and of which some generally reach the centre. Compound corals, again (fig. 44), consist of a greater or less number of structures similar in structure to the above, but united together in different ways into a common mass. Simple corals, therefore, are the Fig. 43
Fig. 43.—Zaphrentis Stokesi, a simple "cup-coral," Upper Silurian, Canada. (After Billings.) Fig. 44
Fig. 44.—Upper surface of a mass of Strombodes pentagonus. Upper Silurian, Canada. (After Billings.) skeletons of single and independent polypes; whilst compound corals are the skeletons of assemblages or colonies of similar polypes, living united with one another another as an organic community.

In the general details of their structure, the Lower Silurian Corals do not differ from the ordinary Corals of the present day. The latter, however, have the vertical calcareous plates of the coral ("septa") arranged in multiples of six or five; whereas the former have these structures arranged in multiples of four, and often showing a cross-like disposition. For this reason, the common Lower Silurian Corals are separated to form a distinct group under the name of Rugose Corals or Rugosa. They are further distinguished by the fact that the cavity of the coral ("visceral chamber") is usually subdivided by more or less numerous horizontal calcareous plates or partitions, which divide the coral into so many tiers or storeys, and which are known as the "tabulæ" (fig. 45).

In addition to the Rugose Corals, the Lower Silurian rocks contain a number of curious compound corals, the tubes of which have either no septa at all or merely rudimentary ones, but which have the transverse partitions or "tabulæ" very highly developed. These are known as the Tabulate Corals; and recent researches on some of their existing allies (such as Heliopora) have shown that they are really allied to the modern Sea-pens, Organ-pipe Corals, and Red Coral, rather than to the typical stony Corals. Amongst the characteristic Rugose Corals of the Lower Silurian Fig. 45
Fig. 45.—Columnaria alveolata, a Rugose compound coral, with imperfect septa, but having the corallites partitioned off into storeys by "tabulæ." Lower Silurian, Canada. (After Billings.) may be mentioned species belonging to the genera Columnaria, Favistella, Streptelasma, and Zaphrentis; whilst amongst the "Tabulate" Corals, the principal forms belong to the genera Chœtetes, Halysites (the Chain-coral), Constellaria, and Heliolites. These groups of the Corals, however, attain a greater development at a later period, and they will be noticed more particularly hereafter.

[Footnote 13: The genus Caryocrinus is sometimes regarded as properly belonging to the Crinoids, but there seem to be good reasons for rather considering it as an abnormal form of Cystidean.]

Passing onto higher animals, we find that the class of the Echinodermata is represented by examples of the Star-fishes (Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct group of the Cystideans (Cystoidea), with one or two of the Brittle-stars (Ophiuroidea)—the Sea-urchins (Echinoidea) being still wanting. The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of. In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly entitled to the name of "Crinoidal limestones," being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12). Allied to the Crinoids are the singular creatures which are known as Cystideans (fig. 46). These are generally composed of a globular or ovate body (the "calyx"), supported upon a short stalk (the "column"), by which the organism was usually attached to some foreign body. The body was enclosed by closely-fitting calcareous plates, accurately jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane. The Fig. 46
Fig. 46.—Group of Cystideans. A, Caryocrinus ornatus,[13] Upper Silurian, America; B, Pleurocystites squamosus, showing two short "arms," Lower Silurian, Canada; C, Pseudocrinus bifasciatus, Upper Silurian, England; D, Lepadocrinus Gebhartii, Upper Silurian, America. (After Hall, Billings, and Salter.) chief distinction which strikes one in comparing the Cystideans with the Crinoids is, that the latter are always furnished, as will be subsequently seen, with a beautiful crown of branched and feathery appendages, springing from the summit of the calyx, and which are composed of innumerable calcareous plates or joints, and are known as the "arms." In the Cystideans, on the other hand, there are either no "arms" at all, or merely short, unbranched, rudimentary arms. The Cystideans are principally, and indeed nearly exclusively, Silurian fossils; and though occurring in the Upper Silurian in no small numbers, they are pre-eminently characteristic of the Llandeilo-Caradoc period of Lower Silurian time. They commenced their existence, so far as known, in the Upper Cambrian; and though examples are not absolutely unknown in later periods, they are pre-eminently characteristic of the earlier portion of the Palæozoic epoch.

The Ringed Worms (Annelides) are abundantly represented in the Lower Silurian, but principally by tracks and burrows similar in essential respects to those which occur so commonly in the Cambrian formation, and calling for no special comment. Much more important are the Articulate animals, represented as heretofore, wholly by the remains of the aquatic group of the Fig. 47
Fig. 47.—Lower Silurian Crustaceans. a, Asaphus tyrannus, Upper Llandeilo; b. Ogygia Buchii, Upper Llandeilo; c, Trinucleus concentricus, Caradoc; d, Caryocaris Wrightii, Arenig (Skiddaw Slates); e, Beyrichia complicata, natural size and enlarged, Upper Llandeilo and Caradoc; f, Primitia strangulata, Caradoc: g. Head-shield of Calymene Blumenbachii, var. brevicapitata, Caradoc; h, Head-shield of Triarthrus Becki (Utica Slates), United States: i, Shield of Leperditia Canadensis, var. Josephiana, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the front. (After Salter, M'Coy, Rupert Jones, and Dana.) Crustaceans. Amongst these are numerous little bivalved forms—such as species of Primitia (fig. 47, f), Beyrichia (fig. 47, e), and Leperditia (fig. 47, i and j). Most of these are very small, varying from the size of a pin's head up to that of a hemp seed; but they are sometimes as large as a small bean (fig. 47, i), and they are commonly found in myriads together in the rock. As before said, they belong to the same great group as the living Water-fleas (Ostracoda). Besides these, we find the pod-shaped head-shields of the shrimp-like Phyllopods—such as Caryocaris (fig. 47, d) and Ceratiocaris. More important, however, than any of these are the Trilobites, which may be considered as attaining their maximum development in the Lower Silurian. The huge Paradoxides of the Cambrian have now disappeared, and with them almost all the principal and characteristic "primordial" genera, save Olenus and Agnostus. In their place we have a great number of new forms—some of them, like the great Asaphus tyrannus of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera—Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illœnus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis, and Homalonotus, a few of them passing upwards under new forms into the Upper Silurian.

Coming next to the Mollusca, we find the group of the Sea-mosses and Sea-mats (Polyzoa) represented now by quite a number of forms. Amongst these are examples of the true Lace-corals (Retepora and Fenestella), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to shells and corals (Hippothoa, Alecto, &c.); but perhaps the most characteristic forms belong to the genus Ptilodictya (figs. 48 and 49). In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides. Each of these little chambers or "cells" was originally tenanted by a minute animal, and the whole thus constituted a compound organism or colony.

The Lamp-shells or Brachiopods are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of "Age of Brachiopods" has with justice been applied to the Silurian period as a whole. It would be impossible here to enter into details as to the many Fig. 48
Fig. 48.—Ptilodictya falciformis. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged. Trenton Limestone and Cincinnati Group, America. (Original.) Fig. 49
Fig. 49.—A, Ptilodictya acuta; B. Ptilodictya Schafferi. a, Fragment, of the natural size; b, Portion, enlarged to show the cells. Cincinnati Group of Ohio and Canada. (Original.) different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera Orthis, Strophomena, and Leptœna for illustration, as being specially characteristic of this period, Fig. 50
Fig. 50.—Lower Silurian Brachiopods. a and a', Orthis biforata, Llandeilo-Caradoc, Britain and America: b, Orthis flabellulum, Caradoc, Britain: c, Orthis subquadrata, Cincinnati Group, America; c', Interior of the dorsal valve of the same: d, Strophomena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.) though not exclusively confined to it. The numerous shells which belong to the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes, Fig. 51
Fig. 51.—Lower Silurian Brachiopods, a, Strophomena alternata, Cincinnati Group, America; b, Strophomena filitexta, Trenton and Cincinnati Groups, America; c, Orthis testudinaria, Caradoc, Europe, and America; d, d', Orthis plicateila, Cincinnati Group, America; e, e', e'', Leptœna sericea, Llandeilo and Caradoc, Europe and America. (After Meek, Hall, and the Author.) generally more or less convex, and marked with radiating ribs or lines. The valves of the shell are united to one another by teeth and sockets, and there is a straight hinge-line. The beaks are also separated by a distinct space ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, passed a muscular cord attaching the shell to some foreign object. The genus Strophomena (fig. 50, d, and 51, a and b) is very like Orthis in general character; but the shell is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate. In Leptœna, again (fig. 51, e), the shell is like Strophomena in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave. Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and Athyris; but none of these can claim the importance to which the three previously-mentioned groups are entitled.

The remaining Lower Silurian groups of Mollusca can be but briefly glanced at here. The Bivalves (Lamellibranchiata) find numerous representatives, belonging to such genera as Modiolopsis, Ctenodonta, Orthonota, Palœarca, Lyrodesma, Fig. 52
Fig. 52.—Murchisonia gracilis, Trenton Limestone, America. (After Billings.) Ambonychia,and Cleidophorus. The Univalves (Gasteropoda) are also very numerous, the two most important genera being Murchisonia (fig. 52) and Pleurotomaria. In both these groups the outer lip of the shell is notched; but the shell in the former is elongated and turreted, whilst in the latter it is depressed. The curious oceanic Univalves known as the Heteropods are also very abundant, the principal forms belonging to Bellerophon and Maclurea. In the former (fig. 53) there is a symmetrical convoluted shell, like that of the Pearly Nautilus in shape, but without any internal partitions, and having the aperture often expanded and notched behind. The species of Maclurea (fig. 54) are found both in North America and in Scotland, and are exclusively confined to the Lower Silurian period, so far as known. They have the shell coiled into a flat spiral, the mouth being furnished with a very curious, thick, and solid lid or "operculum." The Lower Silurian Pteropods, or "Winged snails," are numerous, and belong principally to the genera Theca, Conularia, and Tentaculites, the last-mentioned of these often being extremely abundant in certain strata.