The Ancient Life History of the Earth Part 20

[Ill.u.s.tration: Fig. 186.--Cretaceous Angiosperms. a. _Sa.s.safras Cretaceum; b, Liriodendron Meekii; c, Leguminosites Marcoua.n.u.s; d, Salix Meekii_. (After Dana.)]

As regards animal life, the _Protozoans_ of the Cretaceous period are exceedingly numerous, and are represented by _Foraminifera_ and _Sponges_. As we have already seen, the White Chalk itself is a deep-sea deposit, almost entirely composed of the microscopic sh.e.l.ls of _Foraminifers_, along with Sponge-spicules, and organic _debris_ of different kinds (see fig. 7). The green grains which are so abundant in several minor subdivisions of the Cretaceous, are also in many instances really casts in glauconite of the chambered sh.e.l.ls of these minute organisms. A great many species of _Foraminifera_ have been recognised in the Chalk; but the three princ.i.p.al genera are _Globigerina, Rotalia_ (fig. 187), and _Textularia_--groups which are likewise characteristic of the "ooze" of the Atlantic and Pacific Oceans at great depths.

The flints of the Chalk also commonly contain the sh.e.l.ls of _Foraminifera_. The Upper Greensand has yielded in considerable numbers the huge _Foraminifera_ described by Dr Carpenter under the name of _Parkeria_, the spherical sh.e.l.ls of which are composed of sand-grains agglutinated together, and sometimes attain a diameter of two and a quarter inches. The Cretaceous Sponges are extremely numerous, and occur under a great number of varieties of shape and structure; but the two most characteristic genera are _Siphonia_ and _Ventriculites_, both of which are exclusively confined to strata of this age. The _Siphonioe_ (fig. 188) consist of a pear-shaped, sometimes lobed head, supported by a longer or shorter stern, which breaks up at its base into a number of root-like processes of attachment. The water gained access to the interior of the Sponge by a number of minute openings covering the surface, and ultimately escaped by a single, large, chimney-shaped aperture at the summit. In some respects these sponges present a singular resemblance to the beautiful "Vitreous Sponges" (_Holtenia_ or _Pheronema_) of the deep Atlantic; and, like these, they were probably denizens of a deep sea, The _Ventriculites_ of the Chalk (fig. 189) is, however, a genus still more closely allied to the wonderful flinty Sponges, which have been shown, by the researches of the Porcupine, Lightning, and Challenger expeditions, to live half buried in the Calcareous ooze of the abysses of our great oceans. Many forms of this genus are known, having "usually the form of graceful vases, tubes, or funnels, variously ridged or grooved, or otherwise ornamented on the surface, frequently expanded above into a cup-like lip, and continued below into a bundle of fibrous roots. The minute structure of these bodies shows an extremely delicate tracery of fine tubes, sometimes empty, sometimes filled with loose calcareous matter dyed with peroxide of iron."--(Sir Wyville Thomson.) Many of the Chalk sponges, originally calcareous, have been converted into flint subsequently; but the Ventriculites are really composed of this substance, and are therefore genuine "Siliceous Sponges," like the existing Venus's Flower-Basket (_Euplectella_). Like the latter, the skeleton was doubtless originally composed, in the young state, of disconnected six-rayed spicules, which ultimately become fixed together to const.i.tute a continuous frame-work. The sea-water, as in the recent forms, must have been admitted to the interior of the Sponge by numerous apertures on its exterior, subsequently escaping by a single large opening at its summit.

[Ill.u.s.tration: Fig. 187--_Kotalia Boueana_.]

[Ill.u.s.tration: Fig. 188.--_Siphonia ficus. Upper Greensand. Europe.]

[Ill.u.s.tration: Fig. 189.--_Ventriculites simplex_. White Chalk.

Britain.]

Amongst the _Coelenterates_, the "Hydroid Zoophytes" are represented by a species of the encrusting genus _Hydractinia_, the h.o.r.n.y polypary of which is so commonly found at the present day adhering to the exterior of sh.e.l.ls. The occurrence of this genus is of interest, because it is the first known instance in the entire geological series of the occurrence of an unquestionable Hydroid of a modern type, though many of the existing forms of these animals possess structures which are perfectly fitted for preservation in the fossil condition. The corals of the Cretaceous series are not very numerous, and for the most part are referable to types such as _Trochocyathus, Stephanophyllia, Parasmilia, Synhelia_ (fig.

190), &c., which belong to the same great group of corals as the majority of existing forms. We have also a few "Tabulate Corals"

(_Polytremacis_), hardly, if at all, generically separable from very ancient forms (_Heliolites_); and the Lower Greensand has yielded the remains of the little _Holocystis elegans_, long believed to be the last of the great Palaeozoic group of the _Rugosa_.

[Ill.u.s.tration: Fig. 190.--_Synhelia Sharpeana_. Chalk, England.]

As regards the _Echinoderms_, the group of the _Crinoids_ now exhibits a marked decrease in the number and variety of its types.

The "stalked" forms are represented by _Pentacrinus_ and _Bourgueticrinus_, and the free forms by Feather-stars like our existing _Comatuloe_; whilst a link between the stalked and free groups is const.i.tuted by the curious "Tortoise Encrinite (_Marsupites_). By far the most abundant Cretaceous Echinoderms, however, are Sea-urchins (_Echinoids_); though several Star-fishes are known as well. The remains of Sea-urchins are so abundant in various parts of the Cretaceous series, especially in the White Chalk, and are often so beautifully preserved, that they const.i.tute one of the most marked features of the fauna of the period. From the many genera of Sea-urchins which occur in strata of this age, it is difficult to select characteristic types; but the genera _Galerites_ (fig. 191), _Discoidea_ (fig. 192), _Micraster, Ananchytes, Diadema, Salenia_, and _Cidaris_, may be mentioned as being all important Cretaceous groups.

Coming to the _Annulose Animals_ of the Cretaceous period, there is little special to remark. The _Crustaceans_ belong for the most part to the highly-organised groups of the Lobsters and the Crabs (the Macrurous and Brachyurous Decapods); but there are also numerous little _Ostracodes_, especially in the fresh-water strata of the Wealden. It should further be noted that there occurs here a great development of the singular _Crustaceous_ family of the Barnacles (_Lepadidoe_), whilst the allied family of the equally singular Acorn-sh.e.l.ls (_Balanidoe_) is feebly represented as well.

[Ill.u.s.tration: Fig. 191.--_Galerites albogalerus_, viewed from below, from the side, and from above. White Chalk.]

[Ill.u.s.tration: Fig. 192.--_Discoidea cylindrica_; under, side, and upper aspect. Upper Greensand.]

Pa.s.sing on to the _Mollusca_, the cla.s.s of the Sea-mats and Sea-mosses (_Polyzoa_) is immensely developed in the Cretaceous period, nearly two hundred species being known to occur in the Chalk. Most of the Cretaceous forms belong to the family of the _Escharidoe_, the genera _Eschara_ and _Escharina_ (fig. 193) being particularly well represented. Most of the Cretaceous _Polyzoans_ are of small size, but some attain considerable dimensions, and many simulate Corals in their general form and appearance.

The Lamp-sh.e.l.ls (_Brachiopods_) have now reached a further stage of the progressive decline, which they have been undergoing ever since the close of the Palaeozoic period. Though individually not rare, especially in certain minor subdivisions of the series, the number of generic types has now become distinctly diminished, the princ.i.p.al forms belonging to the genera _Terebratula, Terebratella_ (fig. 194), _Terebratulina, Rhynchonella_, and _Crania_ (fig. 195). In the last mentioned of these, the sh.e.l.l is attached to foreign bodies by the substance of one of the valves (the ventral), whilst the other or free valve is more or less limpet-shaped. All the above-mentioned genera are in existence at the present day; and one _species_--namely, _Terebratulina striata_--appears to be undistinguishable from one now living--the _Terebratulina caputserpentis_.

[Ill.u.s.tration: Fig. 193.--A small fragment of _Escharina Oceani_, of the natural size; and a portion of the same enlarged. Upper Greensand.]

[Ill.u.s.tration: Fig. 194.--_Terebratella Astieriana_. Gault.]

Whilst the Lamp-sh.e.l.ls are slowly declining, the Bivalves (_Limellibranchs_) are greatly developed, and are amongst the most abundant and characteristic fossils of the Cretaceous period.

In the great river-deposit of the Wealden, the Bivalves are forms proper to fresh water, belonging to the existing River-mussels (_Unio_), _Cyrena_ and _Cyclas_; but most of the Cretaceous Lamellibranchs are marine. Some of the most abundant and characteristic of these belong to the great family of the Oysters (_Ostreidoe_). Amongst these are the genera _Gryphtoea_ and _Exogyra_, both of which we have seen to occur abundantly in the Jura.s.sic; and there are also numerous true Oysters (_Ostrea_, fig. 196) and Th.o.r.n.y Oysters (_Spondylus_, fig. 197). The genus _Trigonia_, so characteristic of the Mesozoic deposits in general, is likewise well represented in the Cretaceous strata. No single genus of Bivalves is, however, so highly characteristic of the Cretaceous period as _Inoceramus_, a group belonging to the family of the Pearl-mussels (_Aviculidoe_). The sh.e.l.ls of this genus (fig. 198) have the valves unequal in size, the larger valve often being much twisted, and both valves being marked with radiating ribs or concentric furrows. The hinge-line is long and straight, with numerous pits for the attachment of the ligament which serves to open the sh.e.l.l. Some of the _Inocerami_ attain a length of two or three feet, and fragments of the sh.e.l.l are often found perforated by boring Sponges. Another extraordinary family of Bivalves, which is exclusively confined to the Cretaceous rocks, is that of the _Hippuritidoe_. All the members of this group (fig. 199) were attached to foreign objects, and lived a.s.sociated in beds, like Oysters. The two valves of the sh.e.l.l are always altogether unlike in sculpturing, appearance, shape, and size; and the cast of the interior of the sh.e.l.l is often extremely unlike the form of the outer surface. The type-genus of the family is _Hippurites_ itself (fig. 199), in which the sh.e.l.l is in the shape of a straight or slightly-twisted horn, sometimes a foot or more in length, const.i.tuted by the attached lower valve, and closed above by a small lid-like free upper valve. About a hundred species of the family of the _Hippuritidoe_ are known, all of these being Cretaceous, and occurring in Britain (one species only), in Southern Europe, the West Indies, North America, Algeria, and Egypt. Species of this family occur in such numbers in certain compact marbles in the south of Europe, of the age of the Upper Cretaceous (Lower Chalk), as to have given origin to the name of "Hippurite Limestones," applied to these strata.

[Ill.u.s.tration: Fig. 195.--_Crania Ignabergensis_. The left-hand figure shows the perfect sh.e.l.l, attached by its ventral valve to a foreign body; the middle figure shows the exterior of the limpet-shaped dorsal valve; and the right-hand figure represents the interior of the attached valve. White Chalk.]

[Ill.u.s.tration: Fig. 196.--_Ostrea Couloni_. Lower Greensand.]

[Ill.u.s.tration: Fig. 197.--_Spondylus spinosus_. White Chalk.]

[Ill.u.s.tration: Fig. 198.--_Inoceramus sulcatus_. Gault.]

The Univalves (_Gasteropods_) of the Cretaceous period are not very numerous, nor particularly remarkable. Along with species of the persistent genus _Pleurotomaria_ and the Mesozoic _Nerinoea_, we meet with examples of such modern types as _Turritella_ and _Natica_, the Staircase-sh.e.l.ls (_Solarium_), the Wentle-traps (_Scalaria_), the Carrier-sh.e.l.ls (_Phorus_), &c. Towards the close of the Cretaceous period, and especially in such transitional strata as the Maestricht beds, the Faxoe Limestone, and the Pisolitic Limestone of France, we meet with a number of carnivorous ("siphonostomatous") Univalves, in which the mouth of the sh.e.l.l is notched or produced into a ca.n.a.l. Amongst these it is interesting to recognise examples of such existing genera as the Volutes (_Voluta_, fig. 200), the Cowries (_Cyproea_), the Mitre-sh.e.l.ls (_Mitra_), the Wing - sh.e.l.ls (_Strombus_), the Scorpion-sh.e.l.ls (_Pteroceras_), &c.

[Ill.u.s.tration: Fig. 199.--_Hippurites Toucasiana_. A large individual, with two smaller ones attached to it. Upper Cretaceous, South of Europe.]

[Ill.u.s.tration: Fig. 200.--_Voluta elongata_. White Chalk.]

Upon the whole, the most characteristic of all the Cretaceous Molluscs are the _Cephalopods_, represented by the remains of both _Tetrabranchiate_ and _Dibranchiate_ forms. Amongst the former, the long-lived genus _Nautilus_ (fig. 201) again reappears, with its involute sh.e.l.l, its capacious body-chamber, its simple septa between the air-chambers, and its nearly or quite central siphuncle. The majority of the chambered _Cephalopods_ of the Cretaceous belong, however, to the complex and beautiful family of the _Ammonitidoe_, with their elaborately folded and lobed septa and dorsally-placed siphuncle. This family disappears wholly at the close of the Cretaceous period; but its approaching extinction, so far from being signalised by any slow decrease and diminution in the number of specific or generic types, seems to have been attended by the development of whole series of new forms. The genus _Ammonites_ itself, dating from the Carboniferous, has certainly pa.s.sed its prime, but it is still represented by many species, and some of these attained enormous dimensions (two or three feet in diameter). The genus _Ancyloceras_ (fig.

202), though likewise of more ancient origin (Jura.s.sic), is nevertheless very characteristic of the Cretaceous. In this genus the first portion of the sh.e.l.l is in the form of a flat spiral, the coils of which are not in contact; and its last portion is produced at a tangent, becoming ultimately bent back in the form of a crosier. Besides these pre-existent types, the Cretaceous rocks have yielded a great number of entirely new forms of the _Ammonitidoe_, which are not known in any deposits of earlier or later date. Amongst the more important of these may be mentioned _Crioceras, Turrilites, Scaphites, Hamites, Ptychoceras_, and _Baulites_. In the genus _Crioceras_ (fig. 204, d), the sh.e.l.l consists of an open spiral, the volutions of which are not in contact, thus resembling a partially-unrolled _Ammonite_ or the inner portion of an _Ancyloceras_. In _Turrilites_ (fig. 203), the sh.e.l.l is precisely like that of the _Ammonite_ in its structure; but instead of forming a flat spiral, it is coiled into an elevated turreted sh.e.l.l, the whorls of which are in contact with one another.

In the genus _Scaphites_ (fig. 204, e), the sh.e.l.l resembles that of _Ancyloceras_ in consisting of a series of volutions coiled into a flat spiral, the last being detached from the others, produced, and ultimately bent back in the form of a crosier; but the whorls of the enrolled part of the sh.e.l.l are in contact, instead of being separate as in the latter. In the genus _Hamites_ (fig. 204, f), the sh.e.l.l is an extremely elongated cone, which is bent upon itself more than once, in a hook-like manner, all the volutions being separate. The genus _Ptychoteras_ (fig. 204, a) is very like _Hamites_, except that the sh.e.l.l is only bent once; and the two portions thus bent are in contact with one another. Lastly, in the genus _Baculites_ (fig. 204, b and c) the sh.e.l.l is simply a straight elongated cone, not bent in any way, but possessing the folded septa which characterise the whole Ammonite family. The _Baculite_ is the simplest of all the forms of the _Ammonitidoe_; and all the other forms, however complex, may be regarded as being simply produced by the bending or folding of such a conical septate sh.e.l.l in different ways. The _Baculite_, therefore, corresponds, in the series of the _Ammonitidoe_, to the _Orthoceras_ in the series of the _Nautilidoe_. All the above-mentioned genera are characteristically, or exclusively, Cretaceous, and they are accompanied by a number of other allied forms, which cannot be noticed here. Not a single one of these genera, further, has. .h.i.therto been detected in any strata higher than the Cretaceous. We may therefore consider that these wonderful, varied, and elaborate forms of _Ammonitidoe_ const.i.tute one of the most conspicuous features in the life of the Chalk period.

[Ill.u.s.tration: Fig. 201.--Different views of _Nautilus Danicus_.

Faxoe Limestone (Upper Cretaceous), Denmark.]

[Ill.u.s.tration: Fig. 202.--_Ancyloceras Matheronia.n.u.s_. Gault.]

The _Dibranchiate Cephalopods_ are represented partly by the beak-like jaws of unknown species of Cuttle-fishes and partly by the internal skeletons of Belemnites. Amongst the latter, the genus _Belemnites_ itself holds its place in the lower part of the Cretaceous series; but it disappears in the upper portion of the series, and its place is taken by the nearly-allied genus _Belemnitella_ (fig. 205), distinguished by the possession of a straight fissure in the upper end of the guard. This also disappears at the close of the Cretaceous period; and no member of the great Mesozoic family of the _Belemnitidoe_ has. .h.i.therto been discovered in any Tertiary deposit, or is known to exist at the present day.

[Ill.u.s.tration: Fig. 203.--_Turrilites catenatus_. The lower figure represents the entire sh.e.l.l; the upper figure represents the base of the sh.e.l.l seen from below. Gault.]

[Ill.u.s.tration: Fig. 204.--a, _Ptychoceras Emericianum_, reduced--Lower Greensand; b, _Baculites anceps_, reduced--Chalk; c, Portion of the same, showing the folded edges of the septa; d, _Crioceras cristatum_, reduced--Gault; e, _Scaphites oequalis_, natural size--Chalk; f, _Hamites rotundus_, restored--Gault.]

Pa.s.sing on next to the _Vertebrate Animals_ of the Cretaceous period, we find the _Fishes_ represented as before by the Ganoids and the Placoids, to which, however, we can now add the first known examples of the great group of the _Bony Fishes_ or _Teleosteans_, comprising the great majority of existing forms.

The _Ganoid_ fishes of the Cretaceous (_Lepidotus, Pycnodus_, &c.) present no features of special interest. Little, also, need be said about the _Placoid_ fishes of this period. As in the Jura.s.sic deposits, the remains of these consist partly of the teeth of genuine Sharks (_Lamna, Odontaspis_, &c.) and partly of the teeth and defensive spines of Cestracionts, such as the living Port-Jackson Shark. The pointed and sharp-edged teeth of true Sharks are very abundant in some beds, such as the Upper Greensand, and are beautifully preserved. The teeth of some forms (_Carcharias_, &c.) attain occasionally a length of three or four inches, and indicate the existence in the Cretaceous seas of huge predaceous fishes, probably larger than any existing Sharks.

The remains of _Cestracionts_ consist partly of the flattened teeth of genera such as _Acrodus_ and _Ptychodus_ (the latter confined to rocks of this age), and partly of the pointed teeth of _Hybodus_, a genus which dates from the Trias. In this genus the teeth (fig. 206) consist of a princ.i.p.al central cone, flanked by minor lateral cones; and the fin-spines (fig. 207) are longitudinally grooved, and carry a series of small spines on their hinder or concave margin. Lastly, the great modern order of the Bony Fishes or _Teleosteans_ makes its first appearance in the Upper Cretaceous rocks, where it is represented by forms belonging to no less than three existing groups--namely, the Salmon family (_Salmonidoe_), the Herring family (_Clupeidoe_), and the Perch family (_Percidoe_). All these fishes have thin, h.o.r.n.y, overlapping scales, symmetrical ("h.o.m.ocercal") tails, and bony skeletons. The genus _Beryx_ (fig. 208, 1) is one represented by existing species at the present day, and belongs to the Perch family. The genus _Osmeroides_, again (fig. 208, 2), is supposed to be related to the living Smelts (_Osmerus_), and, therefore, to belong to the Salmon tribe.

[Ill.u.s.tration: Fig. 205.--Guard of _Belemnitella mucronata_. White Chalk.]

[Ill.u.s.tration: Fig. 206.--Tooth of _Hybodus_.]

[Ill.u.s.tration: Fig. 207.--Fin-spine of _Hybodus_. Lower Greensand.]

[Ill.u.s.tration: Fig. 208.--1, _Beryx Lewesiensis_, a Percoid fish from the Chalk; 2, _Osmeroides Mantelli_, a Salmonoid fish from the Chalk.]

No remains of _Amphibians_ have hitherto been detected in any part of the Cretaceous series; but _Reptiles_ are extremely numerous, and belong to very varied types. As regards the great extinct groups of Reptiles which characterise the Mesozoic period as a whole, the huge "Enaliosaurs" or "Sea-Lizards" are still represented by the _Ichthyosaur_ and the _Plesiosaur_. Nearly allied to the latter of these is the _Elasmosaurus_ of the American Cretaceous, which combined the long tail of the Ichthyosaur with the long neck of the Plesiosaur. The length of this monstrous Reptile could not have been less than fifty feet, the neck consisting of over sixty vertebrae and measuring over twenty feet in length. The extraordinary Flying Reptiles of the Jura.s.sic are likewise well represented in the Cretaceous rocks by species of the genus _Pterodactylus_ itself, and these later forms are much more gigantic in their dimensions than their predecessors. Thus some of the Cretaceous Pterosaurs seem to have had a spread of wing of from twenty to twenty-five feet, more than realising the "Dragons" of fable in point of size. The most remarkable, however, of the Cretaceous _Pterosaurs_ are the forms which have recently been described by Professor Marsh under the generic t.i.tle of _Pteranodon_. In these singular forms--so far only known as American--the animal possessed a skeleton in all respects similar to that of the typical Pterodactyles, except that the jaws are completely dest.i.tute of teeth. There is, therefore, the strongest probability that the jaws were encased in a h.o.r.n.y sheath, thus coming to resemble the beak of a Bird. Some of the recognised species of _Pteranodon_ are very small; but the skull of one species (_P. Longiceps_) is not less than a yard in length, and there are portions of the skull of another species which would indicate a length of four feet for the cranium. These measurements would point to dimensions larger than those of any other known Pterosaurs.

The great Mesozoic order of the _Deinosaurs_ is largely represented in the Cretaceous rocks, partly by genera which previously existed in the Jura.s.sic period, and partly by entirely new types. The great delta-deposit of the Wealden, in the Old World, has yielded the remains of various of these huge terrestrial Reptiles, and very many others have been found in the Cretaceous deposits of North America. One of the most celebrated of the Cretaceous Deinosaurs is the _Iguanodon_, so called from the curious resemblance of its teeth to those of the existing but comparatively diminutive _Iguana_. The teeth (fig. 209) are soldered to the inner face of the jaw, instead of being sunk in distinct sockets; and they have the form of somewhat flattened prisms, longitudinally ridged on the outer surface, with an obtusely triangular crown, and having the enamel crenated on one or both sides. They present the extraordinary feature that the crowns became worn down flat by mastication, showing that the _Iguanodon_ employed its teeth in actually chewing and triturating the vegetable matter on which it fed. There can therefore be no doubt but that the _Iguanodon_, in spite of its immense bulk, was an herbivorous Reptile, and lived princ.i.p.ally on the foliage of the Cretaceous forests amongst which it dwelt. Its size has been variously estimated at from thirty to fifty feet, the thigh-bone in large examples measuring nearly five feet in length, with a circ.u.mference of twenty-two inches in its smallest part. With the strong and ma.s.sive hind-limbs are a.s.sociated comparatively weak and small fore-limbs; and there seems little reason to doubt that the _Iguanodon_ must have walked temporarily or permanently upon its hind-limbs, after the manner of a Bird. This conjecture is further supported by the occurrence in the strata which contain the bones of the _Iguanodon_ of gigantic three-toed foot-prints, disposed _singly_ in a double track. These prints have undoubtedly been produced by some animal walking on two legs; and they can hardly, with any probability, be ascribed to any other than this enormous Reptile. Closely allied to the _Iguanodon_ is the _Hadrosaurus_ of the American Cretaceous, the length of which is estimated at twenty-eight feet. _Iguanodon_ does not appear to have possessed any integumentary skeleton; but the great _Hyloeosaurus_ of the Wealden seems to have been furnished with a longitudinal crest of large spines running down the back, similar to that which is found in the comparatively small Iguanas of the present day. The _Megalosaurus_ of the Oolites continued to exist in the Cretaceous period; and, as we have previously seen, it was carnivorous in its habits. The American _Loelaps_ was also carnivorous, and, like the Megalosaur, which it very closely resembles, appears to have walked upon its hind-legs, the fore-limbs being disproportionately small.

[Ill.u.s.tration: Fig. 209.--Teeth of Iguanodon Mantellii. Wealden, Britain.]

Another remarkable group of Reptiles, exclusively confined to the Cretaceous series, is that of the _Mosasauroids_, so called from the type-genus _Mosasaurus_. The first species of _Mosasaurus_ known to science was the _M. Camperi_ (fig. 210), the skull of which--six feet in length--was discovered in 1780 in the Maestricht Chalk at Maestricht. As this town stands on the river Meuse, the name of _Mosasaurus_ ("Lizard of the Meuse") was applied to this immense Reptile. Of late years the remains of a large number of Reptiles more or less closely related to _Mosasaurus_, or absolutely belonging to it, have been discovered in the Cretaceous deposits of North America, and have been described by Professors Cope and Marsh. All the known forms of this group appear to have been of large size--one of them, _Mosasaurus princeps_, attaining the length of seventy-five or eighty feet, and thus rivalling the largest of existing Whales in its dimensions. The teeth in the "Mosasauroids" are long, pointed, and slightly curved; and instead of being sunk in distinct sockets, they are firmly amalgamated with the jaws, as in modern Lizards. The palate also carried teeth, and the lower jaw was so constructed as to allow of the mouth being opened to an immense width, somewhat as in the living Serpents. The body was long and snake-like, with a very long tail, which is laterally compressed, and must have served as a powerful swimming-apparatus. In addition to this, both pairs of limbs have the bones connecting them with the trunk greatly shortened; whilst the digits were enclosed in the integuments, and const.i.tuted paddles, closely resembling in structure the "flippers" of Whales and Dolphins. The neck is sometimes moderately long, but oftener very short, as the great size and weight of the head would have led one to antic.i.p.ate. Bony plates seem in some species to have formed an at any rate partial covering to the skin; but it is not certain that these integumentary appendages were present in all. Upon the whole, there can be no doubt but that the Mosasauroid Reptiles--the true "Sea-serpents" of the Cretaceous period--were essentially aquatic in their habits, frequenting the sea, and only occasionally coming to the land.

[Ill.u.s.tration: Fig. 210.--Skull of _Mosasaurus Camperi_, greatly reduced. Maestricht Chalk.]

The "Mosasauroids" have generally been regarded as a greatly modified group of the Lizards (_Lacertilia_). Whether this reference be correct or not--and recent investigations render it dubious--the Cretaceous rocks have yielded the remains of small Lizards not widely removed from existing forms. The recent order of the _Chelonians_ is also represented in the Cretaceous rocks, by forms closely resembling living types. Thus the fresh-water deposits of the Wealden have yielded examples of the "Terrapins" or "Mud-Turtles"

(_Emys_); and the marine Cretaceous strata have been found to contain the remains of various species of Turtles, one of which is here figured (fig. 211). No true Serpents (_Ophidia_) have as yet been detected in the Cretaceous rocks; and this order does not appear to have come into existence till the Tertiary period. Lastly, true Crocodiles are known to have existed in considerable numbers in the Cretaceous period. The oldest of these occur in the fresh-water deposit of the Wealden; and they differ from the existing forms of the group in the fact that the bodies of the vertebrae, like those of the Jura.s.sic Crocodiles, are bi-concave, or hollowed out at both ends. In the Greensand of North America, however, occur the remains of Crocodiles which agree with all the living species in having the bodies of the vertebrae in the region of the back hollowed out in front and convex behind.

[Ill.u.s.tration: Fig. 211.--Carapace of _Chelone Benstedi_. Lower Chalk. (After Owen.)]

_Birds_ have not hitherto been shown, with certainty, to have existed in Europe during the Cretaceous period, except in a few instances in which fragmentary remains belonging to this cla.s.s have been discovered. The Cretaceous deposits of North America have, however, been shown by Professor Marsh to contain a considerable number of the remains of Birds, often in a state of excellent preservation. Some of these belong to Swimming or Wading Birds, differing in no point of special interest from modern birds of similar habits. Others, however, exhibit such extraordinary peculiarities that they merit more than a pa.s.sing notice. One of the forms in question const.i.tutes the genus _Ichthyornis_ of Marsh, the type-species of which (_I. Dispar_) was about as large as a Pigeon. In two remarkable respects, this singular Bird differs from all known living members of the cla.s.s.

One of these respects concerns the jaws, both of which exhibit the Reptilian character of being armed with numerous small pointed _teeth_ (fig. 212, a), sunk in distinct sockets. No existing bird possesses teeth; and this character forcibly recalls the Bird-like Pterosaurs, with their toothed jaws. _Ichthyornis_, however, possessed fore-limbs constructed strictly on the type of the "wing" of the living Birds; and it cannot, therefore, be separated from this cla.s.s. Another extraordinary peculiarity of _Ichthyornis_ is, that the bodies of the _vertebrie_ (fig.

212, c) were _bi-concave_, as is the case with many extinct Reptiles and almost all Fishes, but as does not occur in any living Bird. There can be little doubt that _Ichthyornis_ was aquatic in its habits, and that it lived princ.i.p.ally upon fishes; but its powerful wings at the same time indicate that it was capable of prolonged flight. The tail of _Ichthyornis_ has, unfortunately, not been discovered; and it is at present impossible to say whether this resembled the tail of existing Birds, or whether it was elongated and composed of separate vertebrae, as in the Jura.s.sic _Archoeopteryx_.

Still more wonderful than _Ichthyornis_ is the marvellous bird described by Marsh under the name of _Hesperornis regalis_. This presents us with a gigantic diving bird, somewhat resembling the existing "Loons" (_Colymbus_), but agreeing with _Ichthyornis_ in having the jaws furnished with conical, recurved, pointed teeth (fig. 212, b). Hence these forms are grouped together in a new sub-cla.s.s, under the name of _Odontornithes_ or "Toothed Birds." The teeth of _Hesperornis_ (fig. 212, d) resemble those of _Ichthyornis_ in their general form; but instead of being sunk in distinct sockets, they are simply implanted in a deep continuous groove in the bony substance of the jaw. The front of the upper jaw does not carry teeth, and was probably encased in a h.o.r.n.y beak. The breast-bone is entirely dest.i.tute of a central ridge or keel, and the wings are minute and quite rudimentary; so that _Hesperornis_, unlike _Ichthyornis_, must have been wholly deprived of the power of flight, in this respect approaching the existing Penguins. The tail consists of about twelve vertebrae, of which the last three or four are amalgamated to form a flat terminal ma.s.s, there being at the same time clear indications that the tail was capable of up and down movement in a vertical plane, this probably fitting it to serve as a swimming-paddle or rudder. The legs were powerfully constructed, and the feet were adapted to a.s.sist the bird in rapid motion through the water. The known remains of _Hesperornis regalis_ prove it to have been a swimming and diving bird, of larger dimensions than any of the aquatic members of the cla.s.s of Birds with which we are acquainted at the present day. It appears to have stood between five and six feet high, and its inability to fly is fully compensated for by the numerous adaptations of its structure to a watery life. Its teeth prove it to have been carnivorous in its habits, and it probably lived upon fishes. It is a curious fact that two Birds agreeing with one another in the wholly abnormal character of possessing teeth, and in other respects so entirely different, should, like _Ichthyornis_ and _Hesperornis_, have lived not only in the same geological period, but also in the same geographical area; and it is equally curious that the area inhabited by these toothed Birds should at the same time have been tenanted by winged and bird-like Reptiles belonging to the toothed genus _Pterodactylus_ and the toothless genus _Pteranodon_.