Fig. 205.—Guard of Belemnitella mucronata. disappears at the close of the Cretaceous period; and no member of the great Mesozoic family of the Belemnitidœ 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
Fig. 206.—Tooth of Hybodus.
Fig. 207.—Fin-spine of Hybodus. Lower Greensand. 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 (Salmonidœ), the Herring family (Clupeidœ), and the Perch family (Percidœ). All these fishes have thin, horny, overlapping scales, symmetrical
Fig. 208.—1, Beryx Lewesiensis, a Percoid fish from the Chalk; 2, Osmeroides Mantelli, a Salmonoid fish from the Chalk. ("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