Page:The Osteology of the Reptiles.pdf/218

200 More usually the feet are pentedactylate, but certain early forms have but three digits, while other later ones may have as many as nine. It is a question whether three was the primitive number, and that all above that number are accessory; more probably the hypodactyly occurred after the ichthyosaur paddle was essentially evolved. Some of these accessory digits seem to have arisen at the sides of the paddles; others by a splitting of the digits, as shown in Figure 158. The paddles of both the plesiosaurs and ichthyosaurs were oar-like and flexible; the feet of the mosasaurs were webbed, more like the feet of ducks and frogs.

In crawling reptiles (Figs. 1, 128) the feet are directed more outwardly, and the motion of the foot upon the epipodials is largely lateral. The structure of the feet in the early forms, even as late as Sauranodon (Fig. 153 ), with a large astragalus and calcaneum, shows an extensive lateral movement of the foot upon the leg in locomotion. In the modern lizards (Fig. 140 ) and Sphenodon (Fig. 139 ) the angle between the leg and foot in locomotion is acute, probably much more so than in the early forms, and this may account for the coössification of the heel bones. In such reptiles the toes always are and must be long, with the main axis of the foot more postaxial. On the other hand, the direction of the foot in the turtles, and especially the tortoises (Fig. 145), is more forward than lateral; the digits of the feet on the two sides are brought more nearly parallel in locomotion. The same acute angle between the foot and leg and the elevation of the heel bones have also resulted in the firmer ossification of the tarsal bones and their fusion. In such locomotion long toes would be a hindrance, and they have been shortened, both by a reduction in the numbers and by a shortening of the segments.

Doubtless this same more mammal-like or turtle-like mode of progression was characteristic of the Dromasauria, Anomodontia, and Theriodontia, and likewise resulted in the reduction of the phalanges and shortening of the toes. One can imagine the difficulties of locomotion if our toes were six inches long! The Cotylosauria have short and broad feet, and many of the later ones, like Pariasaurus and Telerpeton have the astragalus and calcaneum fused. Possibly the mode of progression was more turtle-like than lizard-like, and the results began to be seen in the reduced phalangeal formula of Pariasaurus. Except among the Sauropoda and Stegosauria, in which the