The Ancient Life History of the Earth Part 12

[Ill.u.s.tration: Fig. 108.--_Odontopteris Schlotheimii_. Carboniferous, Europe and North America.]

[Ill.u.s.tration: Fig. 109.--_Calamites cannoeformis_. Carboniferous Rocks, Europe and North America.]

In accordance with M. Brongniart's generalisation, that the Palaeozoic period is, botanically speaking, the "Age of Acrogens," we find the Carboniferous plants to be still mainly referable to the Flowerless or "Cryptogamous" division of the vegetable kingdom.

The flowering or "Phanerogamous" plants, which form the bulk of our existing vegetation, are hardly known, with certainty, to have existed at all in the Carboniferous era, except as represented by trees related to the existing Pines and Firs, and possibly by the Cycads or "false palms."[18] Amongst the "Cryptogams," there is no more striking or beautiful group of Carboniferous plants than the _Ferns_. Remains of these are found all through the Carboniferous, but in exceptional numbers in the Coal-measures, and include both herbaceous forms like the majority of existing species, and arborescent forms resembling the living Tree-ferns of New Zealand. Amongst the latter, together with some new types, are examples of the genera _Psaronius_ and _Caulopteris_, both of which date from the Devonian. The simply herbaceous ferns are extremely numerous, and belong to such widely-distributed and largely-represented genera as _Neuropteris, Odontopteris_ (fig. 108), _Alethopteris, Pecopteris, Sphenopteris, Hymenophyllites_, &c.

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

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

[Ill.u.s.tration: Fig. 110.--_Lepidodendron Sternbergii_, Carboniferous, Europe. The central figure represents a portion of the trunk with its branches, much reduced in size. The right-hand figure is a portion of a branch with the leaves partially attached to it; and the left-hand figure represents the end of a branch bearing a cone of fructification.]

The _Lepidodendroids_, represented mainly by the genus _Lepidodendron_ itself (fig. 110), were large tree-like plants, which attain their maximum in the Carboniferous period, but which appear to commence in the Upper Silurian, are well represented in the Devonian, and survive in a diminished form into the Permian.

The trunks of the larger species of _Lepidodendron_ at times reach a length of fifty feet and upwards, giving off branches in a regular bifurcating manner. The bark is marked with numerous rhombic or oval scars, arranged in quincunx order, and indicating the points where the long, needle-shaped leaves were formerly attached. The fruit consisted of cones or spikes, carried at the ends of the branches, and consisting of a central axis surrounded by overlapping scales, each of which supports a "spore-case"

or seed-vessel. These cones have commonly been described under the name of _Lepidostrobi_. In the structure of the trunk there is nothing comparable to what is found in existing trees, there being a thick bark surrounding a zone princ.i.p.ally composed of "scalariform" vessels, this in turn enclosing a large central pith. In their general appearance the _Lepidodendra_ bring to mind the existing Araucarian Pines; but they are true "Cryptogams,"

and are to be regarded as a gigantic extinct type of the modern Club-mosses (_Lycopodiaceoe_). They are amongst the commonest and most characteristic of the Carboniferous plants; and the majority of the "spore-cases" so commonly found in the coal appear to have been derived from the cones of Lepidodendroids.

The so-called _Sigillanoids_, represented mainly by _Sigillaria_ itself (fig. 111), were no less abundant and characteristic of the Carboniferous forests than the _Lepidodendra_. They commence their existence, so far as known, in the Devonian period, but they attain their maximum in the Carboniferous; and--unlike the Lepidodendroids--they are not known to occur in the Permian period.

They are comparatively gigantic in size, often attaining a height of from thirty to fifty feet or more; but though abundant and well preserved, great divergence of opinion prevails as to their true affinities. The _name_ of Sigillarioids (Lat. _sigilla_, little seals or images) is derived from the fact that the bark is marked with seal-like impressions or leaf-scars (fig. 111).

[Ill.u.s.tration: Fig. 111.--Fragment of the external surface of _Sigillaria Groeseri_, showing the ribs and leaf-scars. The left-hand figure represents a small portion enlarged. Carboniferous, Europe.]

Externally, the trunks of _Sigillaria_ present strong longitudinal ridges, with vertical alternating rows of oval leaf-scars indicating the points where the leaves were originally attached. The trunk was furnished with a large central pith, a thick outer bark, and an intermediate woody zone,--composed, according to Dawson, partly of the disc-bearing fibres so characteristic of Conifers; but, according to Carruthers, entirely made up of the "scalariform"

vessels characteristic of Cryptogams. The size of the pith was very great, and the bark seems to have been the most durable portion of the trunk. Thus we have evidence that in many cases the stumps and "stools" of _Sigillarioe_, standing upright in the old Carboniferous swamps, were completely hollowed out by internal decay, till nothing but an exterior sh.e.l.l of bark was left. Often these hollow stumps became ultimately filled up with sediment, sometimes enclosing the remains of galley-worms, land-snails, or Amphibians, which formerly found in the cavity of the trunk a congenial home; and from the sandstone or shale now filling such trunks some of the most interesting fossils of the Coal-period have been obtained. There is little certainty as to either the leaves or fruits of _Sigillaria_, and there is equally little certainty as to the true botanical position of these plants. By Princ.i.p.al Dawson they are regarded as being probably flowering plants allied to the existing "false palms"

or "_Cycads_," but the high authority of Mr Carruthers is to be quoted in support of the belief that they are Cryptogamic, and most nearly allied to the Club-mosses.

[Ill.u.s.tration: Fig. 112.--_Stigmaria ficoides_. Quarter natural size. Carboniferous.]

Leaving the botanical position of _Sigillaria_ thus undecided, we find that it is now almost universally conceded that the fossils originally described under the name of _Stigmaria_ are the _roots_ of _Sigillaria_, the actual connection between the two having been in numerous instances demonstrated in an unmistakable manner.

The _Stigmarioe_ (fig. 112) ordinarily present themselves in the form of long, compressed or rounded fragments, the external surface of which is covered with rounded pits or shallow tubercles, each of which has a little pit or depression in its centre. From each of these pits there proceeds, in perfect examples, a long cylindrical rootlet; but in many cases these have altogether disappeared. In their internal structure, _Stigmaria_ exhibits a central pith surrounded by a sheath of scalariform vessels, the whole enclosed in a cellular envelope. The _Stigmarioe_ are generally found ramifying in the "under-clay," which forms the floor of a bed of coal, and which represents the ancient soil upon which the _Sigillarioe_ grew.

[Ill.u.s.tration: Fig. 113.--_Trigonocarpon ovatum_. Coal-measures, Britain. (After Liudley and Hutton.)]

The _Lepidodendroids and Sigillaroids, though the first were certainly, and the second possibly, Cryptogamic or flowerless plants, must have const.i.tuted the main ma.s.s of the forests of the Coal period; but we are not without evidence of the existence at the same time of genuine "trees," in the technical sense of this term--namely, flowering plants with large woody stems. So far as is certainly known, all the true trees of the Carboniferous formation were _Conifers_, allied to the existing Pines and Firs.

They are recognised by the great size and concentric woody rings of their prostrate, rarely erect trunks, and by the presence of disc-bearing fibres in their wood, as demonstrated by the microscope; and the princ.i.p.al genera which have been recognised are _Dadoxylon, Paloeoxylon, Araucarioxylon_, and _Pinites_.

Their fruit is not known with absolute certainty, unless it be represented, as often conjectured, by _Trigonocarpon_ (fig. 113).

The fruits known under this name are nut-like, often of considerable size, and commonly three- or six-angled. They probably originally possessed a fleshy envelope; and if truly referable to the _Conifers_, they would indicate that these ancient evergreens produced berries instead of cones, and thus resembled the modern Yews rather than Pines. It seems, further, that the great group of the _Cycads_, which are nearly allied to the _Conifers_, and which attained such a striking prominence in the Secondary period, probably commenced its existence during the Coal period; but these antic.i.p.atory forms are comparatively few in number, and for the most part of somewhat dubious affinities.

CHAPTER XIII.

THE CARBONIFEROUS PERIOD--Continued.

ANIMAL LIFE OF THE CARBONIFEROUS.

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

[Ill.u.s.tration: Fig. 114.--Transparent slice of Carboniferous Limestone, from Spergen Hill, Indiana, U.S., showing numerous sh.e.l.ls of _Endothyra_ (_Rotalia_), _Baiteyi_ slightly enlarged.

(Original.)]

[Ill.u.s.tration: Fig. 115.--_Fusulina cylindrica_, Carboniferous Limestone, Russia.]

The Carboniferous _Protozoans_ consist mainly of _Foraminifera_ and _Sponges_. The latter are still very insufficiently known, but the former are very abundant, and belong to very varied types.

Thin slices of the limestones of the period, when examined by the microscope, very commonly exhibit the sh.e.l.ls of _Foraminifera_ in greater or less plenty. Some limestones, indeed, are made up of little else than these minute and elegant sh.e.l.ls, often belonging to types, such as the Textularians and Rotalians, differing little or not at all from those now in existence. This is the case, for example, with the Carboniferous Limestone of Spergen Hill in Indiana (fig. 114), which is almost wholly made up of the spiral sh.e.l.ls of a species of _Endothyra_. In the same way, though to a less extent, the black Carboniferous marbles of Ireland, and the similar marbles of Yorks.h.i.+re, the limestones of the west of England and of Derbys.h.i.+re, and the great "Scar Limestones" of the north of England, contain great numbers of Foraminiferous sh.e.l.ls; whilst similar organisms commonly occur in the shale-beds a.s.sociated with the limestones throughout the Lower Carboniferous series. One of the most interesting of the British Carboniferous forms is the _Saccammina_ of Mr Henry Brady, which is sometimes present in considerable numbers in the limestones of Northumberland, c.u.mberland, and the west of Scotland, and which is conspicuous for the comparatively large size of its spheroidal or pear-shaped sh.e.l.l (reaching from an eighth to a fifth of an inch in size).

More widely distributed are the generally spindle-shaped sh.e.l.ls of _Fusulina_ (fig. 115), which occur in vast numbers in the Carboniferous Limestone of Russia, Armenia, the Southern Alps, and Spain, similar forms occurring in equal profusion in the higher limestones which are found in the Coal-measures of the United States, in Ohio, Illinois, Indiana, Missouri, &c. Mr Henry Brady, lastly, has shown that we have in the _Nummulina Pristina_ of the Carboniferous Limestone of Namur a genuine _Nummulite_, precursor of the great and important family of the Tertiary Nummulites.

[Ill.u.s.tration: Fig. 116--Corals of the Carboniferous Limestone.

a. _Cyathophyllum paracida_, showing young corallites budded forth from the disc of the old one; a', One of the corallites of the same, seen in cross-section; b, Fragment of a ma.s.s of _Lithostrotion irregulare_; b', One of the corallites of the same, divided transversely; c, Portion of the simple cylindrical coral of _Amplexus coralloides_; c', Transverse section of the same species; d, _Zaphrentis vermicularis_, showing the depression or "fossula" on one side of the cup; e, Fragrent of a ma.s.s of _Syringopora ramulosa_; f, Fragment of _Coetetes tumidus_; f', Portion of the same of the same, enlarged. From the Carboniferous Limestone of Britain and Belgium. (After Thomson, De Koninck, Milne-Edwards and Haime, and the Author.)]

The sub-kingdom of the _Coelenterates_, so far as certainly known, is represented only by _Corals_;[19] but the remains of these are so abundant in many of the limestones of the Carboniferous formation as to const.i.tute a feature little or not at all less conspicuous than that afforded by the Crinoids. As is the case in the preceding period, the Corals belong, almost exclusively, to the groups of the _Rugosa_ and _Tabulata_; and there is a general and striking resemblance and relations.h.i.+p between the coral-fauna of the Devonian as a whole, and that of the Carboniferous. Nevertheless, there is an equally decided and striking amount of difference between these successive faunas, due to the fact that the great majority of the Carboniferous _species_ are new; whilst some of the most characteristic Devonian _genera_ have nearly or quite disappeared, and several new genera now make their appearance for the first time. Thus, the characteristic Devonian types _Heliophyllum, Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia, Endophyllum_, and _Cystiphyllum_, have now disappeared; and the great ma.s.ses of _Favosites_ which are such a striking feature in the Devonian limestones, are represented but by one or two degenerate and puny successors. On the other hand, we meet in the Carboniferous rocks not only with entirely new genera--such as _Axophyllum, Lophophyllum_, and _Londsdaleia_--but we have an enormous expansion of certain types which had just begun to exist in the preceding period. This is especially well seen in the Case of the genus _Lithostrotion_ (fig. 116, b), which more than any other may be considered as the predominant Carboniferous group of Corals. All the species of _Lithostrotion_ are compound, consisting either of bundles of loosely-approximated cylindrical stems, or of similar "coral-lites" closely aggregated together into astraeiform colonies, and rendered polygonal by mutual pressure. This genus has a historical interest, as having been noticed as early as in the year 1699 by Edward Lhwyd; and it is geologically important from its wide distribution in the Carboniferous rocks of both the Old and New Worlds. Many species are known, and whole beds of limestone are often found to be composed of little else than the skeletons of these ancient corals, still standing upright as they grew. Hardly less characteristic of the Carboniferous than the above is the great group of simple "cup-corals," of which _Clisiophyllum_ is the central type. Amongst types which commenced in the Silurian and Devonian, but which are still well represented here, may be mentioned _Syringopora_ (fig. 116, e), with its colonies of delicate cylindrical tubes united at intervals by cross-bars; _Zaphrentis_ (fig. 116, d), with its cup-shaped skeleton and the well-marked depression (or "fossula") on one side of the calice; _Amplexus_ (fig. 116, c), with its cylindrical, often irregularly swollen coral and short septa; _Cyathophyllum_ (fig. 116, a), sometimes simple, sometimes forming great ma.s.ses of star-like corallites; and _Choetetes_, with its branched stems, and its minute, "tabulate" tubes (fig. 116, f). The above, together with other and hardly less characteristic forms, combine to const.i.tute a coral-fauna which is not only in itself perfectly distinctive, but which is of especial interest, from the fact that almost all the varied types of which it is composed disappeared utterly before the close of the Carboniferous period. In the first marine sediments of a calcareous nature which succeeded to the Coal-measures (the magnesian limestones of the Permian), the great group of the _Rugose corals_, which flourished so largely throughout the Silurian, Devonian, and Carboniferous periods, is found to have all but disappeared, and it is never again represented save sporadically and by isolated forms.

[Footnote 19: A singular fossil has been described by Professor Martin Duncan and Mr Jenkins from the Carboniferous rocks under the name of _Paloeocoryne_, and has been referred to the Hydroid Zoophytes (_Corynida_). Doubt, however, has been thrown by other observers on the correctness of this reference.]

[Ill.u.s.tration: Fig. 117.--_Platycrinus tricontadactylus_, Lower Carboniferous. The left-hand figure shows the calyx, arms, and upper part of the stem; and the figure next this shows the surface of one of the joints of the column. The right-hand figure shows the proboscis. (After M'Coy.)]

[Ill.u.s.tration: Fig. 118.--A, _Pentremites pyriformis_, side-view of the body ("calyx"); B, The same viewed from below, showing the arrangement of the plates; C, Body of _Pentremites conoideus_, viewed from above. Carboniferous.]

Amongst the _Echinoderms_, by far the most important forms are the Sea-lilies and the Sea-urchins--the former from their great abundance, and the latter from their singular structure; but the little group of the "Pentremites" also requires to be noticed.

The Sea-lilies are so abundant in the Carboniferous rocks, that it has been proposed to call the earlier portion of the period the "Age of Crinoids." Vast ma.s.ses of the limestones of the period are "crinoidal," being more or less extensively composed of the broken columns, and detached plates and joints of Sea-lilies, whilst perfect "heads" may be exceedingly rare and difficult to procure. In North America the remains of Crinoids are even more abundant at this horizon than in Britain, and the specimens found seem to be commonly more perfect. The commonest of the Carboniferous Crinoids belong to the genera _Cyathocrinus, Actinocrinus, Platycrinus_, (fig. 117), _Poteriocrinus, Zeacrinus_, and _Forbesiocrinus_. Closely allied to the Crinoids, or forming a kind of transition between these and the Cystideans, is the little group of the "Pentremites," or _Blastoids_ (fig. 118).

This group is first known to have commenced its existence in the Upper Silurian, and it increased considerably in numbers in the Devonian; but it was in the seas of the Carboniferous period that it attained its maximum, and no certain representative of the family has been detected in any later deposits. The "Pentremites" resemble the Crinoids in having a cup-shaped body (fig. 118, A) enclosed by closely-fitting calcareous plates, and supported on a short stem or "column," composed of numerous calcareous pieces flexibly articulated together. They differ from the Crinoids, however, in the fact that the upper surface of the body does not support the crown of branched feathery "arms,"

which are so characteristic of the latter. On the contrary, the summit of the cup is closed up in the fas.h.i.+on of a flower-bud, whence the technical name of _Blastoidea_ applied to the group (Gr. _blastos_, a bud; _eidos_, form). From the top of the cup radiate five broad, transversely-striated areas (fig. 118, C), each with a longitudinal groove down its middle; and along each side of each of these grooves there seems to have been attached a row of short jointed calcareous filaments or "pinnules."

[Ill.u.s.tration: Fig. 119.--_Paloechinus ellipticus_, one of the Carboniferous Sea-urchins. The left-hand figure shows one of the "ambulacral areas" enlarged, exhibiting the perforated plates.

The right-land figure exhibits a single plate from one of the "inter-ambulacral areas." (After M'Coy.)]

A few Star-fishes and Brittle-stars are known to occur in the Carboniferous rocks; but the only other Echinodemls of this period which need be noticed are the Sea-urchins (_Echinoids_). Detached plates and spines of these are far from rare in the Carboniferous deposits; but anything like perfect specimens are exceedingly scarce. The Carboniferous Sea-urchins agree with those of the present day in having the body enclosed in a sh.e.l.l formed by an enormous number of calcareous plates articulated together.

The sh.e.l.l may be regarded as, typically, nearly spherical in shape, with the mouth in the centre of the base, and the excretory opening or vent at its summit. In both the ancient forms and the recent ones, the plates of the sh.e.l.l are arranged in ten zones which generally radiate from the summit to the centre of the base.

In five of these zones--termed the "ambulacral areas"--the plates are perforated by minute apertures or "pores," through which the animal can protrude the little water-tubes ("tube-feet") by which its locomotion is carried on. In the other five zones--the so-called "inter-ambulacral areas"--the plates are of larger size, and are not perforated by any apertures. In all the modern Sea-urchins each of these ten zones, whether perforate or imperforate, is composed of two rows of plates; and there are thus twenty rows of plates in all. In the Palaeozoic Sea-urchins, on the other hand, the "ambulacral areas" are often like those of recent forms, in consisting of _two_ rows of perforated plates (fig. 119); but the "inter-ambulacral areas" are always quite peculiar in consisting each of three, four, five, or more rows of large imperforate plates, whilst there are sometimes four or ten rows of plates in the "ambulacral areas" also: so that there are many more than twenty rows of plates in the entire sh.e.l.l. Some of the Palaeozoic Sea-urchins, also, exhibit a very peculiar singularity of structure which is only known to exist in a very few recently-discovered modern forms (viz., _Calveria_ and _Phormosoma_). The plates of the inter-ambulacral areas, namely, overlap one another in an imbricating manner, so as to communicate a certain amount of flexibility to the sh.e.l.l; whereas in the ordinary living forms these plates are firmly articulated together by their edges, and the sh.e.l.l forms a rigid immovable box. The Carboniferous Sea-urchins which exhibit this extraordinary peculiarity belong to the genera _Lepidechinus_ and _Lepidesthes_, and it seems tolerably certain that a similar flexibility of the sh.e.l.l existed to a less degree in the much more abundant genus _Archoeocidaris_. The Carboniferous Sea-urchins, like the modern ones, possessed movable spines of greater or less length, articulated to the exterior of the sh.e.l.l; and these structures are of very common occurrence in a detached condition. The most abundant genera are _Archoeocidaris_ and _Paloechinus_; but the characteristic American forms belong princ.i.p.ally to _Melonites, Oligoporus_, and _Lepidechinus_.

[Ill.u.s.tration: Fig. 120.--_Spirorbis (Microconchus) Carbonarius_, of the natural size, attached to a fossil plant, and magnified.

Carboniferous Britain and North America. (After Dawson.)]

Amongst the _Annelides_ it is only necessary to notice the little spiral tubes of _Spirorbis Carbonarius_ (fig. 120), which are commonly found attached to the leaves or stems of the Coal-plants.

This fact shows that though the modern species of _Spirorbis_ are inhabitants of the sea, these old representatives of the genus must have been capable of living in the brackish waters of lagoons and estuaries.

[Ill.u.s.tration: Fig. 121.--_Prestwichia rotundata_, a Limuloid Crustacean. Coal-measures, Britain. (After Henry Woodward.)]

[Ill.u.s.tration: Fig. 122.--Crustaceans of the Carboniferous Rocks.

a, _Phillipsia seminifera_, of the natural size--Mountain Limestone, Europe; b, One valve of the sh.e.l.l of _Estheria tenella_, of the natural size and enlarged--Coal-measures, Europe; c, Bivalved sh.e.l.l of _Entomoconchus Scouleri_, of the natural size--Mountain Limestone, Europe; d, _Dithyrocaris Scouleri_, reduced in size--Mountain Limestone, Ireland; e, _Paloeocaris typus_, slightly enlarged--Coal-measures, North America; f, _Anthrapaloemon gracilis_, of the natural size--Coal-measures, North America. (After De Koninck, M'Coy, Rupert Jones, and Meek and Worthen.)]

The _Crustaceans_ of the Carboniferous rocks are numerous, and belong partly to structural types with which we are already familiar, and partly to higher groups which come into existence here for the first time. The gigantic _Eurypterids_ of the Upper Silurian and Devonian are but feebly represented, and make their final exit here from the scene of life. Their place, however, is taken by peculiar forms belonging to the allied group of the _Xiphosura_, represented at the present day by the King-crabs or "Horse-shoe Crabs" (_Limulus_). Characteristic forms of this group appear in the Coal-measures both of Europe and America; and though const.i.tuting three distinct genera (_Prestwichia, Belinurus_, and _Euproops_), they are all nearly related to one another. The best known of them, perhaps, is the _Prestwichia rotundala_ of Coalbrookdale, here figured (fig. 121). The ancient and formerly powerful order of the _Trilobites_ also undergoes its final extinction here, not surviving the deposition of the Carboniferous Limestone series in Europe, but extending its range in America into the Coal-measures. All the known Carboniferous forms are small in size and degraded in point of structure, and they are referable to but three genera (_Phillipsia, Griffithides_, and _Brachymetopus_), belonging to a single family. The _Phillipsia seminifera_ here figured (fig. 122, a) is a characteristic species in the Old World. The Water-fleas (_Ostracoaa_) are extremely abundant in the Carboniferous rocks, whole strata being often made up of little else than the little bivalved sh.e.l.ls of these Crustaceans. Many of them are extremely small, averaging about the size of a millet-seed; but a few forms, such as _Entomoconchus Scouleni_ (fig. 122, c), may attain a length of from one to three quarters of an inch. The old group of the _Phyllopods_ is is likewise still represented in some abundance, partly by tailed forms of a shrimp-like appearance, such as _Dithyrocaris_ (fig. 122, d), and partly by the curious striated _Estherioe_ and their allies, which present a curious resemblance to the true Bivalve Molluscs (fig. 122, b). Lastly, we meet for the first time in the Carboniferous rocks with the remains of the highest of all the groups of _Crustaceans_--namely, the so-called "Decapods," in which there are five pairs of walking-limbs, and the hinder end of the body ("abdomen") is composed of separate rings, whilst the anterior end is covered by a head-s.h.i.+eld or "carapace." All the Carboniferous Decapods. .h.i.therto discovered resemble the existing Lobsters, Prawns, and Shrimps (the _Macrura_), in having a long and well-developed abdomen terminated by an expanded tail-fin. The _Paloeocaris typus_ (fig. 122, e) and the _Anthrapaloemon gracilis_ (fig. 122, f), from the Coal-measures of Illinois, are two of the best understood and most perfectly preserved of the few known representatives of the "Long-tailed"

Decapods in the Carboniferous series. The group of the Crabs or "Short-tailed" Decapods (_Brachyura_), in which the abdomen is short, not terminated by a tail-fin, and tucked away out of sight beneath the body, is at present not known to be represented at all in the Carboniferous deposits.

[Ill.u.s.tration: Fig. 123.--_Cyclophthalmus senior_. A fossil Scorpion from the Coal-measures of Bohemia.]

[Ill.u.s.tration: Fig. 124.--_Xylobius Sigillarioe_, a Carboniferous Myriapod. a, A specimen, of the natural size; b, Anterior portion of the same, enlarged; c, Posterior portion, enlarged.