PHYLOGENY. 121 



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 Thero- 

 mora. This approximation is seen in the internal epi- 

 condylar foramen and well-developed condyles of the 

 humerus, and in the well-differentiated seven bones of 

 the tarsus. The epicondylar foramen is only retained 

 in later reptiles in the rhynchocephalian Sphenodon 

 (DoUo); and the condyles of the Dinosauria and all of 

 the other orders, excepting the Ornithosauria and some 

 Lacertilia, are greatly wanting in the strong charac- 

 terization seen in the Theromora. The posterior foot 

 seems to have stamped out the greater part of the tar- 

 sus in the huge Dinosauria, and it is reduced, though 

 to a less degree, in all the other orders. In the paddled 

 Plesiosauria, dwellers in the sea, the tarsus and carpus 

 have lost all characterization, probably by a process of 

 degeneracy, as in the mammalian whales. This is to 

 be inferred from the comparatively late period of their 

 appearance in time. The still more unspecialized 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 (Glauconiidae Peters), while 

 very few (Peropoda) have any trace of a posterior 

 limb. 



The vertebrae are not introduced into the definitions 

 of the orders, since they are not so exclusively distinc- 

 tive as many other parts of the skeleton. They never- 

 theless must not be overlooked. As in the Batrachia, 

 the Permian orders show inferiority in the deficient 

 ossification of the centrum. Many of the Theromora 

 are notochordal, a character not found in any later or- 



