Page:The Osteology of the Reptiles.pdf/174

156 Swimming reptiles with propelling tails [e.g., ichthyosaurs, mosasaurs] have short propodials, sometimes very short; on the other hand, the propodials of limb-propelling water reptiles [e. g., plesiosaurs, proganosaurs] are elongated. The epipodials of ordinary terrestrial 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 considerably 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, 1, 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 reptiles 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 Crocodilia (Figs. 140, 157), unlike other crawling reptiles, and tends to confirm Huene's contestation that the ancestors 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.