1052 CLASS REPTILIA. 



suggests the idea that Reptiles may have been derived from the 

 Amphibians by more than one line of descent. 



The Reptiles in the passage of time have suffered more severely 

 than any other class of the Vertebrata, only four of the above-men- 

 tioned ordinal groups — viz., the Chelonia, Rhynchocephalia, Squa- 

 mata, and Crocodilia — now existing • and the second of these being 

 represented only by a single genus with two species. There is some 

 doubt as to the earliest known appearance of the class, since it has 

 been thought that Mesosaurus (Stereo sternum) may be of Carbon- 

 iferous age, but it is more probably Permian. In undoubted Per- 

 mian we have the Proterosauria, many of the European Anomo- 

 donts, and the Rhynchocephalian genus Palceohatteria ; while 

 many of the American Anomodontia occur in strata which are 

 referred by the Transatlantic geologists to that period. With 

 the advent of the Trias we find all the orders, with the exception 

 of the Ornithosauria and Squamata, more or less fully represented. 

 And while the former order makes its appearance in the succeed- 

 ing Lias, we have at present no traces of the latter till the topmost 

 Jurassic. The class reached, however, its zenith of development 

 in the Jurassic and Cretaceous epochs ; the greatest number 

 of huge aberrant forms being characteristic of the later part of 

 the former and the earlier part of the latter epoch. Although the 

 one existing Rhynchocephalian genus is closely allied to Triassic 

 forms, yet we have no instance among Reptiles of the existence of 

 a genus right up from that period to the present day, as we have in 

 Ceratodus among the Pisces, thus indicating that the higher we 

 ascend in the scale of organisation, the more rapid is the change 

 of types — the same law being exemplified by the occurrence of ex- 

 isting species of Reptiles among the totally extinct Mammals of the 

 Indian Siwaliks. 



