156 THE OSTEOLOGY OF THE REPTILES 



Swimming reptiles with propelling tails [e.g., ichthyosaurs, mosa- 

 saurs] have short propodials, sometimes very short; on the other 

 hand, the propodials of limb-propelling water reptiles [e. g., plesio- 

 saurs, proganosaurs] are elongated. The epipodials of ordinary ter- 

 restrial reptiles are always somewhat shorter than the propodials. 

 Greater shortening of these bones is indicative of swimming habits, 

 possibly also of burrowing; and in strictly aquatic reptiles they are 

 always very short; indeed the degree of water adaptation may be 

 gauged by the proportional lengths of the epipodials. On the other 

 hand, in springing, leaping, or volant reptiles, they may be consider- 

 ably longer than the propodials (Fig. 155). 



The limbs of some Lacertilia and most Ophidia are wholly absent; 

 some snakes have vestiges of the hind pair, and some lizards only 

 vestiges of either pair or the front pair only. All other known reptiles 

 have four functional limbs. 



Primitively {e. g., Figs, i, 128) reptiles were pentadactylate, with 

 the phalangeal formulae 2, 3, 4, 5, 3 for the front, 2, 3, 4, 5, 4 for the 

 hind pair, the fourth digit the longest and strongest; and most rep- 

 tiles still retain these characters. The first digit to be lost is the fifth, 

 and only in a few dinosaurs is the first digit wholly lost. In the more 

 upright-walking kinds, those in which the feet of the two sides are 

 brought more nearly parallel in walking, the greater strength of the 

 foot passes more to the preaxial side, and both the fourth and fifth 

 digits may be obsolete or lost, and very rarely the third also. This 

 weakening of the postaxial digits is especially noticeable in the 

 dinosaurs (Figs. 141, 156) and turtles (Fig. 154), in which the posture 

 in locomotion is more like that of mammals. The same character is 

 also observed in the CrocodiHa (Figs. 140 a, 157), unlike other crawl- 

 ing reptiles, and tends to confirm Huene's contestation that the an- 

 cestors of these reptiles were originally more upright in locomotion 

 than are their descendants. 



As a rule the hind limbs of terrestrial reptiles, as of terrestrial 

 mammals, are more specialized than the anterior ones; that is, there 

 are fewer bones, and the ones remaining are more developed than 

 those of the front feet. Among aquatic and volant reptiles, on the 

 other hand, where locomotion is chiefly effected by the fore limbs, 

 these are more specialized. In certain lizards (Phelsuma) the first 

 digit has become vestigial, the others are well developed. 



