THE LIMBS 



173 



reptiles there is no evidence of a lost prepoUex.^ Theriodesmus (Fig. 

 138 a) of the same group, as restored by the author from Seeley's 

 figures, also lacks the fifth carpale, though the intermedium is not 

 small. The complete carpus is unknown in other members of the 

 Therapsida. 



Sphenodon (Fig. 139 b) of the RhynchocephaUa is the only modern 

 reptile which has retained the primitive structure and arrangement 



Fig. 139. Rhynchocephalian limbs: A, B, Sphenodon. After Howes and Swinnerton. 

 About seven eighths natural size. C, Sauranodon. After Lortet. Nine eighths 

 natural size. D, Pleurosaurus. After Lortet. Nine eighths natural size. 



of the carpal bones. Extinct members of the order and its allies of 

 the Diaptosauria are not sufficiently well known to determine 

 whether this primitive structure is general, though doubtless it has 

 been for the most part. In Rhynchosaurus, in a specimen figured by 

 Newton, traces of the missing bones have been shown in dotted lines, 

 indicating a primitive carpus save for the pisiform which was doubt- 

 less present. 



The carpus of the Crocodilia has been strangely modified (Fig. 

 140 a). It is composed of four bones only in all forms so far as known : 



1 [But see Steiner, Acta Zobl., 1922, pp. 307-360. — Ed.] 



