Chapter 44

THE CRETACEOUS PERIOD.

The next series of rocks in ascending order is the great and important series of the Cretaceous Rocks, so called from the general occurrence in the system of chalk (Lat. _creta_, chalk). As developed in Britain and Europe generally, the following leading subdivisions may be recognised in the Cretaceous series:--

1. Wealden, \_ Lower Cretaceous. 2. Lower Greensand or Neocomian, / 3. Gault, \ 4. Upper Greensand, |_ Upper Cretaceous. 5. Chalk, | 6. Maestricht beds, /

I. _Wealden_.--The _Wealden_ formation, though of considerable importance, is a local group, and is confined to the southeast of England, France, and some other parts of Europe. Its name is derived from the _Weald_, a district comprising parts of Surrey, Sussex, and Kent, where it is largely developed. Its lower portion, for a thickness of from 500 to 1000 feet, is arenaceous, and is known as the Hastings Sands. Its Upper portion, for a thickness of 150 to nearly 300 feet, is chiefly argillaceous, consisting of clays with sandy layers, and occasionally courses of limestone. The geological importance of the Wealden formation is very great, as it is undoubtedly the delta of an ancient river, being composed almost wholly of fresh-water beds, with a few brackish-water and even marine strata, intercalated in the lower portion. Its geographical extent, though uncertain, owing to the enormous denudation to which it has been subjected, is nevertheless great, since it extends from Dorsetshire to France, and occurs also in North Germany. Still, even if it were continuous between all these points, it would not be larger than the delta of such a modern river as the Ganges. The river which produced the Wealden series must have flowed from an ancient continent occupying what is now the Atlantic Ocean; and the time occupied in the formation of the Wealden must have been very great, though we have, of course, no data by which we can accurately calculate its duration.

The fossils of the Wealden series are, naturally, mostly the remains of such animals as we know at the present day as inhabiting rivers. We have, namely, fresh-water Mussels (_Unio_), River-snails (_Paludina_), and other fresh-water shells, with numerous little bivalved Crustaceans, and some fishes.

II. _Lower Greensand_ (_Néocomien_ of D'Orbigny).--The Wealden beds pass upward, often by insensible gradations, into the Lower Greensand. The name Lower Greensand is not an appropriate one, for green sands only occur sparingly and occasionally, and are found in other formations. For this reason it has been proposed to substitute for Lower Greensand the name _Neocomian_, derived from the town of Neufchâtel--anciently called _Neocomum_--in Switzerland. If this name were adopted, as it ought to be, the Wealden beds would be called the Lower Neocomian.

The Lower Greensand or Neocomian of Britain has a thickness of about 850 feet, and consists of alternations of sands, sandstones, and clays, with occasional calcareous bands. The general colour of the series is dark brown, sometimes red; and the sands are occasionally green, from the presence of silicate of iron.

The fossils of the Lower Greensand are purely marine, and among the most characteristic are the shells of _Cephalopods_.

The most remarkable point, however, about the fossils of the Lower Cretaceous series, is their marked divergence from the fossils of the Upper Cretaceous rocks. Of 280 species of fossils in the Lower Cretaceous series, only 51, or about 18 per cent, pass on into the Upper Cretaceous. This break in the life of the two periods is accompanied by a decided physical break as well; for the Gault is often, if not always, unconformably superimposed on the Lower Greensand. At the same time, the Lower and Upper Cretaceous groups form a closely-connected and inseparable series, as shown by a comparison of their fossils with those of the underlying Jurassic rocks and the overlying Tertiary beds. Thus, in Britain no marine fossil is known to be common to the marine beds of the Upper Oolites and the Lower Greensand; and of more than 500 species of fossils in the Upper Cretaceous rocks, almost everyone died out before the formation of the lowest Tertiary strata, the only survivors being one Brachiopod and a few _Foraminifera_.

III. _Gault_ (_Aptien_ of D'Orbigny).--The lowest member of the Upper Cretaceous series is a stiff, dark-grey, blue, or brown clay, often worked for brick-making, and known as the _Gault_, from a provincial English term. It occurs chiefly in the south-east of England, but can be traced through France to the flanks of the Alps and Bavaria. It never exceeds 100 feet in thickness; but it contains many fossils, usually in a state of beautiful preservation.

IV. _Upper Greensand_ (_Albien_ of D'Orbigny; _Unterquader_ and _Lower Plänerkalk_ of Germany).--The Gault is succeeded upward by the _Upper Greensand_, which varies in thickness from 3 up to 100 feet, and which derives its name from the occasional occurrence in it of green sands. These, however, are local and sometimes wanting, and the name "Upper Greensand" is to be regarded as a _name_ and not a description. The group consists, in Britain, of sands and clays, sometimes with bands of calcareous grit or siliceous limestone, and occasionally containing concretions of phosphate of lime, which are largely worked for agricultural purposes.

V. _White Chalk_.--The top of the Upper Greensand becomes argillaceous, and passes up gradually into the base of the great formation known as the true _Chalk_, divided into the three subdivisions of the chalk-marl, white chalk without flints, and white chalk with flints. The first of these is simply argillaceous chalk, and passes up into a great mass of obscurely-stratified white chalk in which there are no flints (_Turonien_ of D'Orbigny; _Mittelquader_ of Germany). This, in turn, passes up into a great mass of white chalk, in which the stratification is marked by nodules of black flint arranged in layers (_Sénonien_ of D'Orbigny; _Oberquader_ of Germany). The thickness of these three subdivisions taken together is sometimes over 1000 feet, and their geographical extent is very great. White Chalk, with its characteristic appearance, may be traced from the north of Ireland to the Crimea, a distance of about 1140 geographical miles; and, in an opposite direction, from the south of Sweden to Bordeaux, a distance of about 840 geographical miles.

VI. In Britain there occur no beds containing Chalk fossils, or in any way referable to the Cretaceous period, above the true White Chalk with flints. On the banks of the Maes, however, near Maestricht in Holland, there occurs a series of yellowish limestones, of about 100 feet in thickness, and undoubtedly superior to the White Chalk. These _Maestricht beds_ (_Danien_ of D'Orbigny) contain a remarkable series of fossils, the characters of which are partly Cretaceous and partly Tertiary. Thus, with the characteristic Chalk fossils, _Belemnites, Baculites_, Sea-Urchins, &c., are numerous Univalve Molluscs, such as Cowries and Volutes, which are otherwise exclusively Tertiary or Recent.

Holding a similar position to the Maestricht beds, and showing a similar intermixture of Cretaceous forms with later types, are certain beds which occur in the island of Seeland, in Denmark, and which are known as the _Faxöe Limestone_.

Of a somewhat later date than the Maestricht beds is the _Pisolitic Limestone_ of France, which rests unconformably on the White Chalk, and contains a large number of Tertiary fossils along with some characteristic Cretaceous types.

The subjoined sketch-section exhibits the general succession of the Cretaceous deposits in Britain:--

In North America, strata of Lower Cretaceous age are well represented in Missouri, Wyoming, Utah, and in some other areas; but the greater portion of the American deposits of this period are referable to the Upper Cretaceous. The rocks of this series are mostly sands, clays, and limestones--_Chalk_ itself being unknown except in Western Arkansas. Amongst the sandy accumulations, one of the most important is the so-called "marl" of New Jersey, which is truly a "Greensand," and contains a large proportion of glauconite (silicate of iron and potash). It also contains a little phosphate of lime, and is largely worked for agricultural purposes. The greatest thickness attained by the Cretaceous rocks of North America is about 9000 feet, as in Wyoming, Utah, and Colorado. According to Dana, the Cretaceous rocks of the Rocky Mountain territories pass upwards "without interruption into a coal-bearing formation, several thousand feet thick, on which the following Tertiary strata lie unconformably." The lower portion of this "Lignitic formation" appears to be Cretaceous, and contains one or more beds of Coal; but the upper part of it perhaps belongs to the Lower Tertiary. In America, therefore, the lowest Tertiary strata appear to rest conformably upon the highest Cretaceous; whereas in Europe, the succession at this point is invariably an unconformable one. Owing, however, to the fact that the American "Lignitic formation" is a shallow-water formation, it can hardly be expected to yield much material whereby to bridge over the great palæontological gap between the White Chalk and Eocene in the Old World.

Owing to the fact that so large a portion of the Cretaceous formation has been deposited in the sea, much of it in deep water, the _plants_ of the period have for the most part been found special members of the series, such as the Wealden beds, the Aix-la-Chapelle sands, and the Lignitic beds of North America. Even the purely marine strata, however, have yielded plant-remains, and some of these are peculiar and proper to the deep-sea deposits of the series. Thus the little calcareous discs termed "coccoliths," which are known to be of the nature of calcareous sea-weeds (_Algoe_) have been detected in the White Chalk; and the flints of the same formation commonly contain the spore-cases of the microscopic _Desmids_ (the so-called Xanthidia), along with the siliceous cases of the equally diminutive _Diatoms_.

The plant-remains of the Lower Cretaceous greatly resemble those of the Jurassic period, consisting mainly of Ferns, Cycads, and Conifers. The Upper Cretaceous rocks, however, both in Europe and in North America, have yielded an abundant flora which resembles the existing vegetation of the globe in consisting mainly of Angiospermous Exogens and of Monocotyledons.[23] In Europe the plant-remains in question have been found chiefly in certain sands in the neighbourhood of Aix-la-Chapelle, and they consist of numerous Ferns, Conifers (such as _Cycadopteris_), Screw Pines (_Pandanus_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_), and many _Proteaceoe_, some of which are referred to existing genera (_Dryandra, Banksia, Grevillea_, &c.)

[Footnote 23: The "Flowering plants" are divided into the two great groups of the Endogens and Exogens. The _Endogens_ (such as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of growth, and the stem is said to be "endogenous;" the young plant also possesses but a single seed-leaf or "cotyledon." Hence these plants are often simply called "_Monocotyledons_." The _Exogens_, on the other hand, have a true bark; and the stem increases by annual additions to the outside, so that rings of growth are produced. The young plant has two seed-leaves or "cotyledons," and these plants are therefore called "_Dicotyledons_." Amongst the Exogens, the Pines (_Conifers_) and the Cycads have seeds which are unprotected by a seed-vessel, and they are therefore called "_Gymnosperms_." All the other Exogens, including the ordinary trees, shrubs, and flowering plants, have the seeds enclosed in a seed-vessel, and are therefore called "_Angiosperms_." The derivation of these terms will be found in the Glossary at the end of the volume.]

In North America, the Cretaceous strata of New Jersey, Alabama, Nebraska, Kansas, &c., have yielded the remains of numerous plants, many of which belong to existing genera. Amongst these may be mentioned Tulip-trees (_Liriodendron_), Sassafras (fig. 186), Oaks (_Quercus_), Beeches (_Fagus_), Plane-trees (_Platanus_), Alders (_Alnus_), Dog-wood (_Cornus_), Willows (_Salix_), Poplars (_Populus_), Cypresses (_Cupressus_), Bald Cypresses (_Taxodium_), Magnolias, &c. Besides these, however, there occur other forms which have now entirely disappeared from North America--as, for example, species of _Cinnamomum_ and _Araucaria_.

It follows from the above, that the Lower and Upper Cretaceous rocks are, from a botanical point of view, sharply separated from one another. The Palæozoic period, as we have seen, is characterised by the prevalance of "Flowerless" plants (_Cryptogams_), its higher vegetation consisting almost exclusively of Conifers. The Mesozoic period, as a whole, is characterised by the prevalence of the Cryptogamic group of the Ferns, and the Gymnospermic groups of the Conifers and the Cycads. Up to the close of the Lower Cretaceous, no Angiospermous Exogens are certainly known to have existed, and Monocotyledonous plants or Endogens are very poorly represented. With the Upper Cretaceous, however, a new era of plant-life, of which our present is but the culmination, commenced, with a great and apparently sudden development of new forms. In place of the Ferns, Cycads, and Conifers of the earlier Mesozoic deposits, we have now an astonishingly large number of true Angiospermous Exogens, many of them belonging to existing types; and along with these are various Monocotyledonous plants, including the first examples of the great and important group of the Palms. It is thus a matter of interest to reflect that plants closely related to those now inhabiting the earth, were in existence at a time when the ocean was tenanted by Ammonites and Belemnites, and when land and sea and air were peopled by the extraordinary extinct Reptiles of the Mesozoic period.

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 shells of _Foraminifers_, along with Sponge-spicules, and organic _débris_ 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 shells of these minute organisms. A great many species of _Foraminifera_ have been recognised in the Chalk; but the three principal 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 shells of _Foraminifera_. The Upper Greensand has yielded in considerable numbers the huge _Foraminifera_ described by Dr Carpenter under the name of _Parkeria_, the spherical shells 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 constitute 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.

Amongst the _Coelenterates_, the "Hydroid Zoophytes" are represented by a species of the encrusting genus _Hydractinia_, the horny polypary of which is so commonly found at the present day adhering to the exterior of shells. 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 Palæozoic group of the _Rugosa_.

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 constituted 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 constitute 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-shells (_Balanidoe_) is feebly represented as well.

Passing on to the _Mollusca_, the class 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-shells (_Brachiopods_) have now reached a further stage of the progressive decline, which they have been undergoing ever since the close of the Palæozoic period. Though individually not rare, especially in certain minor subdivisions of the series, the number of generic types has now become distinctly diminished, the principal forms belonging to the genera _Terebratula, Terebratella_ (fig. 194), _Terebratulina, Rhynchonella_, and _Crania_ (fig. 195). In the last mentioned of these, the shell 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_.

Whilst the Lamp-shells 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 Jurassic; and there are also numerous true Oysters (_Ostrea_, fig. 196) and Thorny 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 shells 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 shell. Some of the _Inocerami_ attain a length of two or three feet, and fragments of the shell 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 associated in beds, like Oysters. The two valves of the shell are always altogether unlike in sculpturing, appearance, shape, and size; and the cast of the interior of the shell is often extremely unlike the form of the outer surface. The type-genus of the family is _Hippurites_ itself (fig. 199), in which the shell is in the shape of a straight or slightly-twisted horn, sometimes a foot or more in length, constituted 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.

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-shells (_Solarium_), the Wentle-traps (_Scalaria_), the Carrier-shells (_Phorus_), &c. Towards the close of the Cretaceous period, and especially in such transitional strata as the Maestricht beds, the Faxöe Limestone, and the Pisolitic Limestone of France, we meet with a number of carnivorous ("siphonostomatous") Univalves, in which the mouth of the shell is notched or produced into a canal. Amongst these it is interesting to recognise examples of such existing genera as the Volutes (_Voluta_, fig. 200), the Cowries (_Cyproea_), the Mitre-shells (_Mitra_), the Wing - shells (_Strombus_), the Scorpion-shells (_Pteroceras_), &c.

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 shell, 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 passed 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 (Jurassic), is nevertheless very characteristic of the Cretaceous. In this genus the first portion of the shell 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 shell 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 shell is precisely like that of the _Ammonite_ in its structure; but instead of forming a flat spiral, it is coiled into an elevated turreted shell, the whorls of which are in contact with one another. In the genus _Scaphites_ (fig. 204, e), the shell 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 shell are in contact, instead of being separate as in the latter. In the genus _Hamites_ (fig. 204, f), the shell 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 shell 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 shell 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 shell 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 hitherto been detected in any strata higher than the Cretaceous. We may therefore consider that these wonderful, varied, and elaborate forms of _Ammonitidoe_ constitute one of the most conspicuous features in the life of the Chalk period.

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 hitherto been discovered in any Tertiary deposit, or is known to exist at the present day.

Passing 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 Jurassic 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 principal 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, horny, overlapping scales, symmetrical ("homocercal") 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.

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 vertebræ and measuring over twenty feet in length. The extraordinary Flying Reptiles of the Jurassic 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 title 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 destitute of teeth. There is, therefore, the strongest probability that the jaws were encased in a horny 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 Jurassic 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 principally 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 circumference of twenty-two inches in its smallest part. With the strong and massive hind-limbs are associated 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.

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 constituted 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 anticipate. 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.

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 vertebræ, like those of the Jurassic 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 vertebræ in the region of the back hollowed out in front and convex behind.

_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 class 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 passing notice. One of the forms in question constitutes 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 class. 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 class. 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 principally 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 vertebræ, as in the Jurassic _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-class, 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 horny beak. The breast-bone is entirely destitute 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 vertebræ, of which the last three or four are amalgamated to form a flat terminal mass, 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 assist 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 class 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_.

No remains of _Mammals_, finally, have as yet been detected in any sedimentary accumulations of Cretaceous age.

LITERATURE.

The following list comprises some of the more important works and memoirs which may be consulted with reference to the Cretaceous strata and their fossil contents:--

(1) 'Memoirs of the Geological Survey of Great Britain.' (2) 'Geology of England and Wales.' Conybeare and Phillips. (3) 'Geology of Yorkshire,' vol. ii. Phillips. (4) 'Geology of Oxford and the Thames Valley.' Phillips. (5) 'Geological Excursions through the Isle of Wight.' Mantell. (6) 'Geology of Sussex.' Mantell. (7) 'Report on Londonderry,' &c. Portlock. (8) 'Recherches sur le Terrain Crétacé Supérieur de l'Angleterre et de l'Irlande.' Barrois. (9) "Geological Survey of Canada"--'Report of Progress, 1872-73.' (10) 'Geological Survey of California.' Whitney. (11) 'Geological Survey of Montana, Idaho, Wyoming, and Utah.' Hayden and Meek. (12) 'Report on Geology,' &c. (British North American Boundary Commission). G. M. Dawson. (13) 'Manual of Geology.' Dana. (14) 'Lethæa Rossica.' Eichwald. (15) 'Petrefacta Germaniæ.' Goldfuss. (16) 'Fossils of the South Downs.' Mantell. (17) 'Medals of Creation.' Mantell. (18) 'Mineral Conchology.' Sowerby. (19) 'Lethæa Geognostica.' Bronn. (20) 'Malacostracous Crustacea of the British Cretaceous Formation' (Palæontographical Society). Bell. (21) 'Brachiopoda of the Cretaceous Formation' (Palæontographical Society). Davidson. (22) 'Corals of the Cretaceous Formation' (Palæontographical Society). Milne-Edwards and Haime. (23) 'Supplement to the Fossil Corals' (Palæontographical Society). Martin Duncan. (24) 'Echinodermata or the Cretaceous Formation' (Palæontographical Society). Wright. (25) 'Monograph of the Belemnitidæ' (Palæontographical Society). Phillips. (26) 'Monograph of the Trigoniæ' (Palæontographical Society). Lycett. (27) 'Fossil Cirripedes' (Palæontographical Society). Darwin. (28) 'Fossil Mollusca of the Chalk of Britain' (Palæontographical Society). Sharpe. (29) 'Entomostraca of the Cretaceous Formation' (Palæontographical Society). Rupert Jones. (30) 'Monograph of the Fossil Reptiles of the Cretaceous Formation' (Palæontographical Society). Owen. (31) 'Manual of Palæontology.' Owen. (32) 'Synopsis of Extinct Batrachia and Reptilia.' Cope. (33) "Structure of the Skull and Limbs in Mosasauroid Reptiles"--'American Journ. Sci. and Arts, 1872.' Marsh. (34) "On Odontornithes"--'American Journ. Sci. and Arts, 1875.' Marsh. (35) 'Ossemens Fossiles.' Cuvier. (36) 'Catalogue of Ornithosauria.' Seeley. (37) 'Paléontologie Française.' D'Orbigny. (38) 'Synopsis des Echinides fossiles.' Desor. (39) 'Cat. Raisonné des Echinides.' Agassiz and Desor. (40) "Echinoids"--'Decades of the Geol. Survey of Britain.' E. Forbes. (41) 'Paléontologie Française.' Cotteau. (42) 'Versteinerungen der Böhmischen Kreide-formation.' Reuss. (43) "Cephalopoda, Gasteropoda, Pelecypoda, Brachiopoda; &c., of the Cretaceous Rocks of India"--'Palæontologica Indica,' ser. i., iii., v., vi., viii. Stoliczka. (44) "Cretaceous Reptiles of the United States"--'Smithsonian Contributions to Knowledge,' vol. xiv. Leidy. (45) 'Invertebrate Cretaceous, and Tertiary Fossils of the Upper Missouri Country,' 1876. Meek.