sters in sight at once. The Mosasaurs were essentially swimming lizards, with four well-developed paddles, and they had little affinity with modern serpents, to which they have been compared. The species are quite numerous, but they belong to comparatively few genera, of which Mosasaurus, Tylosaurus, Lestosaurus, and Edestosaurus, have alone been identified with certainty. The genus Mosasaurus was first found in Europe. All the known species of the group are Cretaceous.
The Crocodilia are abundant in rocks of Cretaceous age in America, and two distinct types are represented. The older type, which is foreshadowed by Belodon of the Trias, has biconcave vertebras, and shows marked affinities with the genus Teleosaurus, from the Jura of Europe. The best-known genus is Hyposaurus, of which there are several species, all more or less resembling in form the modern gavial of the Ganges. A peculiar intermediate form is seen in Diplosaurus, from the Wealden of the Rocky Mountains. The second type, which now makes its appearance for the first time, has procœlian vertebræ, and in other respects resembles existing crocodiles. The genera described are Bottosaurus, Holops, and Thoracosaurus, none of which, so far as known, pass above the Cretaceous. Of Crocodilia with opisthocœlous vertebræ, America, so far as we know, has none. Specimens similar to those so termed in Europe are not uncommon here, but they pertain to Dinosaurs.
In the Eocene fresh-water beds of the West, Crocodilians are especially abundant, and all, with the exception of Limnosaurus, belong apparently to the genus Crocodilus, although some species show certain points of resemblance to existing alligators. The Miocene lake basins of the same region contain no remains of crocodiles, so far as known, and the Pliocene deposits have afforded only a single species. The Tertiary marine beds of the Atlantic coast contain comparatively few Crocodilian remains, and all are of modern types; the genus Gavialis having one Eocene species, and the alligator being represented only in the latest deposits.
It is worthy of special mention, in this connection, that no true Lacertilia, or Lizards, and no Ophidia, or Serpents, have yet been detected in American Cretaceous beds; although their remains, if present, would hardly have escaped observation in the regions explored. The former will doubtless be found, as several species occur in the Mesozoic of Europe; and perhaps the latter, although the Ophidians are apparently a more modern type. In the Eocene lake-basins of Western America, remains of lizards are very numerous, and indicate species much larger than any existing to-day. Some of these,
- Vertebræ which have centra concave at each end have been conveniently termed amphicœlous; those with a cavity in front and a convexity behind, procœlous; where the position of the concavity and convexity is reversed, they are opisthocœhus.—(Huxley, "Anatomy of Vertebrated Animals.")