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. [Illustration: Fig. 205.--Guard of _Belemnitella mucronata_. White Chalk.] [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 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.