200 THE OSTEOLOGY OF THE REPTILES 



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 hypo- 

 dactyly 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 c. The paddles of both the plesiosaurs and ichthyosaurs 

 were oar-hke and flexible; the feet of the mosasaurs were webbed, 

 more like the feet of ducks and frogs. 



In crawhng reptiles (Figs, i, 128) the feet are directed more out- 

 wardly, 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 b), with a large astragalus and calcaneum, 

 shows an extensive lateral movement of the foot upon the leg in loco- 

 motion. In the modern lizards (Fig. 140 d) and Sphenodon (Fig. 

 139 a) the angle between the leg and foot in locomotion is acute, 

 probably much more so than in the early forms, and this may ac- 

 count for the coossification 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 pro- 

 gression 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 loco- 

 motion 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 Paria- 

 saurus. Except among the Sauropoda and Stegosauria, in which the 



