128 



COMPARATIVE ANATOMY 



teria) l closely resemble Urodeles in the structure of the carpus, 

 although the exact homologies of all the different elements can- 

 not yet be stated with certainty. Five digits are always present 

 in both manus and pes, and in Chelonians traces also of the former 

 possession of an extra finger both on the radial and ulnar side 



("pisiform") are t be seen ( Fi g s - 10 ^ 108 > and 109 )- Tne ^ia 

 and fibula always remain separate. 



In Crocodiles, which, like Anurans, possess no trace of an 

 intermedium, the proximal row of the carpus consists of two hour- 

 glass-shaped bones a larger radiale, and a smaller ulnare (Fig. 

 110). A rudiment of a sixth ray is present on the outer side of 



A 



FiG. 107. CARPUS OF A, Hatteria (Sphenodon) punztatri, AND B, Emydura 

 kre/tii. (After Baur. ) 



R, radius ; U, ulna ; r, radiale ; u, ulnare ; i, intermedium ; c 1 , radial centrale ; 

 -, ulnar centrale; 1-5, carpalia ; p, ulnar sesamoid (pisiform); I-V, the 

 metacarpals. 



the latter. The centrale, as in Anura, comes to be situated in the 

 distal row, which is much less developed than the proximal. 



In all Reptiles the tarsus undergoes a marked reduction, 

 especially in its proximal portion, and gradually leads to the 

 type seen in Birds. Thus in Chelonians and Lizards the proximal 

 tarsals all run together into a single mass which corresponds to the 

 tibiale, intermedium, fibulare, and centrale, and the last mentioned 

 element can no longer be recognised in Lizards, even in the embryo. 

 Traces of an extra radial ray are present. 



In the distal row three or four (five in Palseohatteria) separate 

 tarsals are developed, but these may unite partly with one another 



1 In Hatteria and Chelydra serpentina amongst existing Reptiles, a double 

 centrale is present in the carpus, and traces of a double condition of this element 

 are seen in certain other Chelonians. 



