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198 quadrupedal Sauropoda 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, 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 ) have only the hook-shaped metatarsal left. The greatly elongated feet were adapted more for perching 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 corresponding 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, 153 ), or many of them, and the Sauria (Fig. 140 ) and Chelonia (Figs. 144 , 145 , 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 articulates 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), Dromasauria (Fig. 137 ), Anomodontia, Cynodontia, as in the mammals, 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 (Fig. 145 ) in many tortoises. The river turtles (Trionychoidea. Fig. 154) have normally four phalanges in the fourth and the fifth digits, which may rather be ascribed to a secondary hyperphalangy. More than three phalanges have also been observed in some Pleurodira. The chameleon lizards have the phalangeal formula 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 Dinosauria, as has been mentioned above.

Hyperphalangy and Hyperdactyly. An increase of the phalanges