198 THE OSTEOLOGY OF THE REPTILES 



quadrupedal Sauropoda (i) the axis of the foot is more to the preaxial 

 side; in other dinosaurs it is the third toe that is the stoutest, though 

 less so in the oldest theropods (a, b), this arguing perhaps a more 

 sauropod-like mode of progression. 



The earliest pterodactyls had two or three phalanges in the fifth 

 toe; the later ones (Fig. 155 d) have only the hook-shaped meta- 

 tarsal left. The greatly elongated feet were adapted more for perch- 

 ing or clinging than for locomotion. A striking peculiarity is seen in 

 the greatly reduced second phalange of the third toe and the second 

 and third of the fourth toes, singularly identical with the correspond- 

 ing phalanges of the hand of the therocephalian Scymnognathus (Fig. 

 138) . Similar reduced phalanges are seen in the hand of the theropod 

 Struthiomimus and the tree sloths among mammals, in all cases 

 doubtless to be ascribed to the grasping or clinging habits. 



A peculiarity of the fifth metatarsal among the Diapsida (Figs. 

 139 A, 153 b), or many of them, and the Sauria (Fig. 140 d) and 

 Chelonia (Figs. 144 b, 145 c, 154) is the more or less hook-like shape, 

 proximally, a character which has been adduced in proof of their 

 phylogenetic relationships. In all such cases the metatarsal articu- 

 lates with the fourth tarsale, and the fifth tarsale is absent. In those 

 reptiles which have a fifth tarsale, either ossified or cartilaginous, 

 the metatarsal is straight, and perhaps also in those reptiles in which 

 the foot had become more or less erect or digitigrade before its 

 entire loss. 



Hypophalangy. In the Chelonia (Figs. 144, 145, 154), Droma- 

 sauria (Fig. 137 a, b), Anomodontia, Cynodontia, as in the mam- 

 mals, the primitive phalangeal formula suffered a reduction to 2, 3, 

 3, 3, 3 in both front and hind feet, with a further reduction to 2, 2, 

 2, 2, 2 (i) (Fig. 145 a) in many tortoises. The river turtles (Triony- 

 choidea. Fig. 154) have normally four phalanges in the fourth and 

 the fifth digits, which may rather be ascribed to a secondary hyper- 

 phalangy. More than three phalanges have also been observed in 

 some Pleurodira. The chameleon lizards have the phalangeal for- 

 mula 2, 3, 4, 4, 3 for both fore and hind feet (Fig. 143), and various 

 examples of partial reduction of the postaxial digits occur among the 

 Cotylosauria {Pariasaurus), Crocodilia, and especially the Dino- 

 sauria, as has been mentioned above. 



Hyper phalangy and Hyperdactyly. An increase of the phalanges 



