126 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



The basal podial region, which is nearly typical in some reptiles, 

 urodeles (fig. 137) and man, consists of three rows of bones, a proximal 

 of three bones, a radiale or tibiale on the anterior side, an ulnare or 

 fibulare on the other, and an intermedium between them. The 

 distal row consists of five carpales or tarsales, numbered from the 

 anterior side. 



The third row is composed of one or two centrales between the 

 other rows. The metapodials and the digits, also numbered from 

 one to five, have, in some cases special names, the thumb (digit I) being 

 the pollex, the corresponding great toe being the hallux, the fifth 

 digit being called minimus, the second in the hand, the index, 

 the fourth the annulus. 



From this typical condition all forms of chiropterygia — legs, arms, 

 wings — are derived by modification, fusion and disappearance of 

 parts. The more distal a part the more variable it is; reduction takes 

 place on the margins of the appendage, the axial portions being the 

 last to disappear. Occasionally in various groups (amphibia, mam- 

 mals) there occur what are interpreted as rudimentary additional 

 digits — ^prehallux, prepollex, postminimus — but their status is un- 

 certain. There are also certain membrane bones developed in the 



