522 THE POPULAR SCIENCE MONTHLY. 



sters in sight at once. The Mosasaurs were essentially swimming liz- 

 ards, 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 Ifosasaurus, Tylosaurus, Lestosaurus, and JZdestosmcrus, have 

 alone been identified with certainty. The genus Mosasaums was 

 first found in Europe. All the known species of the group are Cre- 

 taceous. 



The Crocodilia are abundant in rocks of Cretaceous age in Amer- 

 ica, 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 2'eleosaurics, from the Jura of 

 Europe. The best-known genus is Hyposaicrus, 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 Dip>losaurus, 

 from the Wealden of the Rocky Mountains. The second type, which 

 now makes its appearance for the first time, has proccelian 1 vertebra?, 

 and in other respects resembles existing crocodiles. The genera de- 

 scribed are Bottosaurus, Holops, and Thoracosaurus, none of which, so 

 far as known, pass above the Cretaceous. Of Crocodilia with opis- 

 thocoelous x vertebra?, America, so far as we know, has none. Speci- 

 mens 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 espe- 

 cially abundant, and all, with the exception of Limnosaurus, belong 

 apparently to the genus Crocodilus, although some species show cer- 

 tain 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 compara- 

 tively 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 

 JOacertilia, 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 ex- 

 plored. The former will doubtless be found, as several species occur 

 in the Mesozoic of Europe ; and perhaps the latter, although the Ophid- 

 ians are apparently a more modern type. In the Eocene lake-basins 

 of Western America, remains of lizards are very numerous, and indi- 

 cate species much larger than any existing to-day. Some of these, 



1 Vertebra) which have centra concave at each end have been conveniently termed 

 amphicoslous ; those with a cavity in front and a convexity behind, proccelous ; where the 

 position of the concavity and convexity is reversed, they are opistkoccehus. (Huxley, 

 "Anatomy of Vertebrated Animals.") 



