VERTEBRATA: REPTILIA. 



555 



aquatic in their habits. This conjecture has been raised to 

 the rank of a certainty by the discovery that the fore and hind 

 limbs of the Mosasauroids were 

 in the form of fin-like paddles, 

 resembling the flippers of whales 

 in general structure, and in hav- 

 ing the digits distinct and only 

 conjoined by integument (fig. 

 308). There can therefore be 

 no doubt that Mosasaurus like 

 the living Amblyrhynchus was 

 aquatic in its habits, and fre- 

 quented the sea-shore, coming, 

 in fact, only occasionally to the 

 land. Professor Marsh has also 

 recently shown that some species 

 possess bony dermal scutes, thus 

 rendering their Lacertilian affini- 

 ties somewhat dubious. 



Though possessing certain ab- 

 errant characters, it seems best 

 in the meanwhile to regard the 

 Mosasauridcz ( the Pythonomor- 

 pha of Cope) as an extinct group 

 of the Lacertilia. 



ORDER IV. CROCODILIA. 

 The last and highest order of 

 the living Reptilia is that of the 

 Crocodilia, including the living Crocodiles, Alligators, and Gav- 

 ials, and characterised by the following peculiarities : 



The body is covered with an outer epidermic exoskeleton com- 

 posed of horny scales, and an inner dermal exoskeleton consisting 

 of transverse rows of squared bony plates or scutes, which may be 

 confaied to the dorsal surface alone, or may exist on the ventral 

 surface as well, and which are disposed on the back of the neck 

 into groups of different form and number in certain species. The 

 bones of the skull and face are firmly united together, and the two 

 halves or rami of the lower jaw are united in front by a suture. 

 There is a single row of teeth, which are implanted in distinct 

 sockets, and hollowed at the base for the germs of the new 

 teeth, by which they are successively pushed out and replaced 

 during the life of the animal. The centra of the dorsal vertebra 

 in all living Crocodilia are proccelous or concave in front, but 

 in the extinct forms they may be either amphiccelous (con- 

 cave at both ends) or opisthocoelous (concave behind). The 



Fig. 308. Right anterior paddle of Les- 

 tosaurus simus, one -twelfth of the 

 natural size. (After Marsh.) a Scap- 

 ula ; b Coracoid ; c Humerus ; d Ra- 

 dius ; e Ulna. 



