336 THE STEUCTURAL EVIDENCE OF EVOLUTION. 



thrown backward so as to support the viscera in a more posterior 

 position. This is an obvious necessity to a bipedal animal where 

 the vertebral column is not perpendicular, as in birds. And it 

 is from the Dinosauria that the birds are sujiposed to have arisen. 

 (Plates XV and XVI.) The main line of the Eeptilia, however, 

 departs from both the mammalian and the avian type and loses 

 in strength as compared with the former. In the latest orders, 

 the Phythonomorpha and Ojohidia, the pelvis is rudimental or 

 absent. 



As regards the limbs, the degeneracy is well marked. No 

 reptilian order of later ages approaches so near to the Mammalia 

 in these parts as do the Permian Theromorpha. This approxi- 

 mation is seen in the internal epicondylar foramen and well-devel- 

 oped condyles of the humerus, and in the well-differentiated 

 seven bones of the tarsus. The epicondylar foramen is only re- 

 tained in later reptiles in the Ehynchocephalian Ilatteria (Dollo) ; 

 and the condyles of the Dinosauria and all of the other orders, 

 excepting the Ornithosauria and some Lacertilia, are greatly 

 wanting in the strong characterization seen in the TheromorjDJia. 

 The posterior foot seems to have stamped out the greater part of 

 the tarsus in the huge Dinosauria, and it is reduced, though to a 

 less degree, in all the other orders. In the paddled Sauroptery- 

 gia, dwellers in the sea, the tarsus and carpus have lost all char- 

 acterization, probably by a process of degeneracy, as in the mam- 

 malian whales. This is to be inferred from the comparatively 

 late period of their appearance in time. The still more unspecial- 

 ized feet and limbs of the Ichthyosaurus (Ichthyopterygia) can 

 not yet be ascribed to degeneracy, for their history is too little 

 known. At the end of the line, the snakes present us with another 

 evidence of degeneracy. But few have a pelvic arch (Stenostomi- 

 dse Peters), while very few (Peropoda) have any trace of a poste- 

 rior limb. 



The vertebrae are not introduced into the definitions of the 

 orders, since they are not so exclusively distinctive as many other 

 parts of the skeleton. They nevertheless must not be overlooked. 

 As in the Batrachia, the Permian orders show inferiority in the 

 deficient ossification of the centrum. Many of the Theromorpha 

 are notochordal, a character not found in any later order of 

 reptiles excepting in a few Lacertilia (Gecconidse). They thus 

 differ from the Mammalia, whose characters are approached more 

 nearly by some of the terrestrial Dinosauria in this respect. 



