[Illustration:  Fig. 206.—­Tooth of Hybodus.]

[Illustration:  Fig. 207.—­Fin-spine of Hybodus.  Lower Greensand.]

[Illustration:  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 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