The Ancient Life History of the Earth Part 10

[Ill.u.s.tration: Fig. 85.--Fragment of _Favosites hemispherica_, of the natural size. Upper Silurian and Devonian of America.

(After Billings.)]

The commonest, and at the same time the largest, of these are the "honeycomb corals," forming the genus _Favosites_ (figs.

84, 85), which derive both their vernacular and their technical names from their great likeness to ma.s.ses of petrified honeycomb.

The most abundant species are _Favosites Gothlandica_ and _F.

Hemispherica_, both here figured, which form ma.s.ses sometimes not less than two or three feet in diameter. Whilst _Favosites_ has acquired a popular name by its honey-combed appearance, the resemblance of _Michelinia_ to a fossilised wasp's nest with the comb exposed is hardly less striking, and has earned for it a similar recognition from the non-scientific public. In addition to these, there are numerous branching or plant-like Tabulate Corals, often of the most graceful form, which are distinctive of the Devonian in all parts of the world.

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

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

Of the remains of Ringed-worms (_Annelides_), the most numerous and the most interesting are the calcareous envelopes of some small tube-inhabiting species. No one who has visited the seaside can have failed to notice the little spiral tubes of the existing _Spirorbis_ growing attached to sh.e.l.ls, or covering the fronds of the commoner Sea weeds (especially _Fucus serratus_). These tubes are inhabited by a small Annelide, and structures of a similar character occur not uncommonly from the Upper Silurian upwards. In the Devonian rocks, _Spirorbis_ is an extremely common fossil, growing in hundreds attached to the outer surface of corals and sh.e.l.ls, and appearing in many specific forms (figs.

86 and 87); but almost all the known examples are of small size, and are liable to escape a cursory examination.

[Ill.u.s.tration: Fig. 87.--a, _Spirobois omphalodes_, natural size and enlarged. Devonian, Europe and America; b, _Spirorbis Arkonensis_, of the natural size and enlarged; c, The same, with the tube twisted in the reverse direction. Devonian, America.

(Onginal.)]

[Ill.u.s.tration: Fig. 88. a b, _Spirorbis laxus_, enlarged, Upper Silurian, America; c, _Spirorbis spinulifera_, of the natural size and enlarged, Devonian, Canada. (After Hall and the Author.)]

[Ill.u.s.tration: Fig. 88.--Devonian Trilobites; a, _Phacops latifrons_, Devonian of Britain, the Continent of Europe, and South America; b, _Homalonotus armatus_, Europe; c, _Phacops (Trimerocephalus) loevis_, Europe; d, Head-shield of _Phacops (Portlockia) granulatus_, Europe. (After Salter and Burmeister.)]

The _Crustaceans_ of the Devonian are princ.i.p.ally _Eurypterids_ and _Trilobites_. Some of the former attain gigantic dimensions, and the quarrymen in the Scotch Old Red give them the name of "seraphim" from their singular scale-like ornamentation. The _Trilobites_, though still sufficiently abundant in some localites, have undergone a yet further diminution since the close of the Upper Silurian. In both America and Europe quite a number of generic types have survived from the Silurian, but few or no new ones make their appearance during this period in either the Old World or the New. The _species_, however, are distinct; and the princ.i.p.al forms belong to the genera _Phacops_ (fig. 88, a, c, d), _Homalonotus_ (fig. 88, b), _Proetus_, and _Bronteus_.

The species figured above under the name of _Phacops latifrons_ (fig. 88, a), has an almost world-wide distribution, being found in the Devonian of Britain, Belgium, France, Germany, Russia, Spain, and South America; whilst its place is taken in North America by the closely-allied _Phacops rana_. In addition to the _Trilobites_, the Devonian deposits have yielded the remains of a number of the minute _Ostracoda_, such as _Entomis_ ("_Cypridina_"), _Leperditia_, &c., which sometimes occur in vast numbers, as in the so-called "_Cypridina_ Slates" of the German Devonian.

There are also a few forms of _Phyllopods_ (_Estheria_). Taken as a whole, the Crustacean fauna of the Devonian period presents many alliances with that of the Upper Silurian, but has only slight relationships with that of the Lower Carboniferous.

Besides _Crustaceans_, we meet here for the first time with the remains of _air-breathing Articulates_, in the shape of _Insects_.

So far, these have only been obtained from the Devonian rocks of North America, and they indicate the existence of at least four generic types, all more or less allied to the existing May-flies (_Ephemeridoe_). One of these interesting primitive insects, namely, _Platephemera antiqua_ (fig. 89), appears to have measured five inches in expanse of wing; and another (_Xelloneura antiquorum_) has attached to its wing the remains of a "stridulating-organ" similar to that possessed by the modern Gra.s.shoppers--the instrument, as Princ.i.p.al Dawson remarks, of "the first music of living things that Geology as yet reveals to us."

[Ill.u.s.tration: Fig. 89.--Wing of _Platephemera antiqua_ Devonian, America. (After Dawson.)]

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

[Ill.u.s.tration: Fig. 90.--Fragment of _Clathropora intertexta_, of the natural size and enlarged. Devonian, Canada. (Original.)]

[Ill.u.s.tration: Fig. 91.--Fragment of _Ceriopora Hamiltonensis_, of the natural size and enlarged. Devonian, Canada. (Original.)]

[Ill.u.s.tration: Fig. 92.--Fragment of _Fenestella magnifica_, of the natural size and enlarged. Devonian, Canada. (Original.)]

[Ill.u.s.tration: Fig. 93.--Fragment of _Retepora Phillipsi_, of the natural size and enlarged. Devonian, Canada. (Original.)]

[Ill.u.s.tration: Fig. 94.--Fragment of _Fenestella cribrosa_, of the natural size and enlarged. Dovonian, Canada. (Original.)]

The majority of the Devonian _Polyzoa_ belong, however, to the great and important Palaeozoic group of the Lace-corals (_Fenestella_, figs. 92 and 94, _Retepora_, fig. 93, _Polypora_, and their allies).

In all these forms there is a h.o.r.n.y skeleton, of a fan-like or funnel-shaped form, which grew attached by its base to some foreign body. The frond consists of slightly-diverging or nearly parallel branches, which are either united by delicate cross-bars, or which bend alternately from side to side, and become directly united with one another at short intervals--in either case giving origin to numerous oval or oblong perforations, which communicate to the whole plant-like colony a characteristic netted and lace-like appearance. On one of its surfaces--sometimes the internal, sometimes the external--the frond carries a number of minute chambers or "cells," which are generally borne in rows on the branches, and of which each originally contained a minute animal.

[Ill.u.s.tration: Fig. 95.--_Spirifera sculptilis_. Devonian, Canada.

(After Billings.)]

[Ill.u.s.tration: Fig. 96.--_Spirifera mucronata_. Devonian, America.

(After Billings.)]

[Ill.u.s.tration: Fig. 97.--_Atrypa reticularis_. Upper Silurian and Devonian of Europe and America. (After Billings.)]

The _Brachiopods_ still continue to be represented in great force through all the Devonian deposits, though not occurring in the true Old Red Sandstone. Besides such old types as _Orthis, Strophomena, Lingula, Athyris_, and _Rhynchonella_, we find some entirely new ones; whilst various types which only commenced their existence in the Upper Silurian, now undergo a great expansion and development. This last is especially the case with the two families of the _Spiriferidoe_ and the _Produclidoe_. The _Spirifers_, in particular, are especially characteristic of the Devonian, both in the Old and New Worlds--some of the most typical forms, such as _Spirifera mucronata_ (fig. 96), having the sh.e.l.l "winged," or with the lateral angles prolonged to such an extent as to have earned for them the popular name of "fossil-b.u.t.terflies." The closely-allied _Spirifera disjunda_ occurs in Britain, France, Spain, Belgium, Germany, Russia, and China. The family of the _Productidoe_ commenced to exist in the Upper Silurian, in the genus _Chonetes_, and we shall hereafter find it culminating in the Carboniferous in many forms of the great genus _Producta_[17] itself. In the Devonian period, there is an intermediate state of things, the genus _Chonetes_ being continued in new and varied types, and the Carboniferous _Produdoe_ being represented by many forms of the allied group _Productella_.

Amongst other well-known Devonian Brachiopods may be mentioned the two long-lived and persistent types _Atrypa reticularis_ (fig. 97) and _Strophomena rhomboidalis_ (fig. 98). The former of these commences in the Upper Silurian, but is more abundantly developed in the Devonian, having a geographical range that is nothing less than world-wide; whilst the latter commences in the Lower Silurian, and, with an almost equally cosmopolitan range, survives into the Carboniferous period.

[Footnote 17: The name of this genus is often written _Productus_, just as _Spirifera_ is often given in the masculine gender as _Spirifer_ (the name originally given to it). The masculine termination to these names is, however, grammatically incorrect, as the feminine noun _cochlea_ (sh.e.l.l) is in these cases _understood_.]

[Ill.u.s.tration: Fig. 98.--_Strophomena rhomboidalis_. Lower Silurian, Upper Silurian, and Devonian of Europe and America.]

[Ill.u.s.tration: Fig. 99.--Different views of _Platyceras dumosum_, of the natural size. Devonian, Canada. (Original.)]

The Bivalves (_Lamellibranchiata_) of the Devonian call for no special comment, the genera _Pterinea_ and _Megalodon_ being, perhaps, the most noticeable. The Univalves (_Gasteropods_), also, need not be discussed in detail, though many interesting forms of this group are known. The type most abundantly represented, especially in America, is _Platyceras_ (fig. 99), comprising thin, wide-mouthed sh.e.l.ls, probably most nearly allied to the existing "Bonnet-limpets," and sometimes attaining very considerable dimensions. We may also note the continuance of the genus _Euomphalus_, with its discoidal spiral sh.e.l.l. Amongst the _Heteropods_, the survival of _Bellerophon_ is to be recorded; and in the "Winged-snails," or _Pteropods_, we find new forms of the old genera _Tentaculites_ and _Conularia_ (fig. 100).

The latter, with its fragile, conical, and often beautifully ornamented sh.e.l.l, is especially noticeable.

[Ill.u.s.tration: Fig. 100.--_Conularia ornata, of the natural size.

Devonian, Europe.]

[Ill.u.s.tration: Fig. 101.--_Clymenia Sedgwickii_. Devonian, Europe.]

The remains of _Cephalopoda_ are far from uncommon in the Devonian deposits, all the known forms being still Tetrabranchiate. Besides the ancient types _Orthoceras_ and _Cyrtoceras_, we have now a predominance of the spirally-coiled chambered sh.e.l.ls of _Goniat.i.tes_ and _Clymenia_. In the former of these the sh.e.l.l is shaped like that of the _Nautilus_; but the part.i.tions between the chambers ("septa") are more or less lobed, folded, or angulated, and the "siphuncle" runs along the _back_ or convex side of the sh.e.l.l--these being characters which approximate _Goniat.i.tes_ to the true Ammonites of the later rocks. In _Clymenia_, on the other hand, whilst the sh.e.l.l (fig. 101) is coiled into a flat spiral, and the part.i.tions or septa are simple or only slightly lobed, there is still this difference, as compared with the _Nautilus_, that the tube of the siphuncle is placed on the _inner_ or concave side of the sh.e.l.l. The species of _Clymenia_ are exclusively Devonian in their range; and some of the limestones of this period in Germany are so richly charged with fossils of this genus as to have received the name of "Clymenien-kalk."

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

It is to the two last of these groups that the Devonian fishes belong, and they are more specially referable to the _Ganoids_.

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

There can be little doubt, therefore, but that the majority of the Devonian fishes were truly marine in their habits, though it is probable that many of them lived in shallow water, in the immediate neighbourhood of the sh.o.r.e, or in estuaries.

[Ill.u.s.tration: Fig. 102.--Fishes of the Devonian rocks of America.

a, Diagram of the jaws and teeth of _Dinichthys Hertzeri_, viewed from the front, and greatly reduced; b, Diagram of the skull of _Macropetalichthys Sullivanti_, reduced in size; c, A portion of the enamelled surface of the skull of the same, magnified; d, One of the scales of _Onychodus sigmoides_, of the natural size; e, One of the front teeth of the lower jaw of the same, of the natural size: f, Fin-spine of _Mach.o.e.racanthus major_, a shark-like fish, reduced in size. (After Newberry.)]

[Ill.u.s.tration: Fig. 103.--_Cephalaspis Lyellii_. Old Red Sandstone, Scotland. (After Page.)]

[Ill.u.s.tration: Fig. 104.--_Pterichthys cornutus_. Old Red Sandstone, Scotland. (After Aga.s.siz.)]

The Devonian Galloids belong to a number of groups; and it is only possible to notice a few of the most important forms here.

The modern group of the Sturgeons is represented, more or less remotely, by a few Devonian fishes--such as _Asterosteus_; and the great _Macropetalichthys_ of the Corniferous limestone of North America is believed by Newberry to belong to this group. In this fish (fig. 102, b) the skull was of large size, its outer surface being covered with a tuberculated enamel; and, as in the existing Sturgeons, the mouth seems to have been wholly dest.i.tute of teeth. Somewhat allied, also, to the Sturgeons, is a singular group of armoured fishes, which is highly characteristic of the Devonian of Britain and Europe, and less so of that of America.

In these curious forms the head and front extremity of the body were protected by a buckler composed of large enamelled plates, more or less firmly united to one another; whilst the hinder end of the body was naked, or was protected with small scales. Some forms of this group--such as _Pteraspis_ and _Coccosteus_--date from the Upper Silurian; but they attain their maximum in the Devonian, and none of them are known to pa.s.s upwards into the overlying Carboniferous rocks. Amongst the most characteristic forms of this group may be mentioned _Cephalaspis_ (fig. 103) and _Pterichthys_ (fig. 104). In the former of these the head-shield is of a crescentic shape, having its hinder angles produced backwards into long "horns," giving it the shape of a "saddler's knife."

No teeth have been discovered; but the body was covered with small ganoid scales, and there was an unsymmetrical tail-fin.

In _Pterichthys_--which, like the preceding, was first brought to light by the labours of Hugh Miller--the whole of the head and the front part of the body were defended by a buckler of firmly-united enamelled plates, whilst the rest of the body was covered with small scales. The form of the "pectoral fins" was quite unique--these having the shape of two long, curved spines, somewhat like wings, covered by finely-tuberculated ganoid plates.

All the preceding forms of this group are of small size; but few fishes, living or extinct, could rival the proportions of the great _Dinichthys_, referred to this family by Newberry.

In this huge fish (fig. 102, a) the head alone is over three feet in length, and the body is supposed to have been twenty-five or thirty feet long. The head was protected by a ma.s.sive cuira.s.s of bony plates firmly articulated together, but the hinder end of the body seems to have been simply enveloped in a leathery skin. The teeth are of the most formidable description, consisting in both jaws of serrated dental plates behind, and in front of enormous conical tusks (fig. 102, a). Though immensely larger, the teeth of _Dinichthys_ present a curious resemblance to those of the existing Mud-fishes (_Lepidosiren_).

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

[Ill.u.s.tration: Fig. 105.--A, _Polypterus_, a recent Ganoid fish; B, _Osteolepis_, a Devonian Ganoid; a a, Pectoral fins, showing the fin-rays arranged round a central lobe.]

[Illusration: Fig. 106.--_Holoptychius n.o.bilissimus_, restored.

Old Red Sandstone, Scotland. A, Scale of the same.]