'■pisiforme 



radius 



tibio- 



— ulna 



metatarsal 5 



pisiforme 



1 calcified' 



fibula 



Figure 6-50. Manus and pes of Tupinambis, A and B, and Sphenodon, C and D. 



The structure of the carpus in the early reptiles is not 

 clearly revealed, but a captorhinid and the pelycosaurs 

 (Figure 6-51) are quite like Sphenodon. Seymouria appears 

 to have had at least one centrale. The number of digits was 

 five in these reptiles, and the phalangeal formula was 

 2-3-4-5-3. 



Among later reptiles, including the living types, a reduc- 

 tion in the number of carpais or digits has occurred. The 

 alligator lacks an intermedium and has only three distal 

 elements. Carpais I and II tend to fuse, and carpais III and 

 IV are joined from the beginning of ossification. The pisi- 

 forme is much reduced. The ichthyosaurs had paddles or 

 flippers like the whale and in these the number of phalanges 

 is even greater than in the mammal — all five digits are in- 

 volved in this polyphalangy. 



An extreme of the reptile type is observed in the bird, 

 where the carpus includes only a radiale and ulnare and 

 the digits are reduced to three. It has generally been as- 

 sumed that the remaining digits were one, two, and three — 

 a fourth vestigal digit was also known. Several embryologi- 

 cal studies suggest that in fact the well-developed digits are 

 two, three, and four, with five in a vestigial state. 



Pelvic appendage The pelvic girdle of the reptile as ob- 

 served in Sphenodon or Tupinamhis has three elements nearly 



radius 



fibula 



Figure 6-51. Manus and pes of a pelycosaur, Ophiacodon. (After 

 Romer, 1955, 1956). 



BILATERAL APPENDAGES 



175 



