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Rh didactyl, the extreme of specialization among reptiles. In the Theropoda (Fig. 141 ) the thumb is the stoutest digit, its claw the largest. In the herbivorous dinosaurs (Fig. 141 ) the hand is less preaxial, the first and second fingers being the larger. In the Trachodontidae (Fig. 141 ), indeed, the first finger is absent. In all herbivorous forms the outer fingers are reduced, though the fifth is seldom entirely absent, the phalangeal formula never exceeding 2, 3, 4, 3, 2, the claws lacking in the two postaxial digits. In Trachodon a greater reduction has occurred, almost the maximum among reptiles, the formula, according to Lambe, being 0, 3, 3, 2, 2. The ungual phalanges of both front and hind feet are characteristic, curved and sharply pointed in the Theropoda (Fig. 141 ), more obtuse in the Sauropoda (Fig. 141 ), for the most part hoof-like in the Predentata (Fig. 141 ).

The foot or pes of reptiles is similar in structure to the hand, the reduction of the toes being usually anticipatory of the fingers in the terrestrial forms. There was one more phalange in the fifth toe than in the fifth finger primitively. In Pariasaurus, only, of the Cotylosauria, the phalangeal formula is slightly reduced, though primitive in Propappus, a related genus.

The loss of the fifth toe is rare among reptiles, aside from the Dinosauria. The crocodiles (Fig. 157 ) have only the fifth metatarsal left, and the fourth toe has but four phalanges. A very few lizards also have lost the fifth toe. It is often reduced among the Chelonia (Fig. 154 ); usually one, sometimes two, of the normal phalanges are lost. The greater strength of the foot in this order as in the dinosaurs is more to the preaxial side, unlike most other reptiles.

The foot of dinosaurs (Fig. 156), so far as the reduction of phalanges is concerned, is less specialized than the hand, the Theropoda (Fig. 156 ) retaining the original formula, except in the fifth toe. Plateosaurus and Anchisaurus, from the Trias, have the formula 2, 3, 4, 5, 1; Allosaurus , from the lower Cretaceous, and Struthiomimus , from the uppermost Cretaceous, 2, 3, 4, 5, 0, the fifth metatarsal a vestige. The known Sauropoda have 2, 3, 4, 3, 1 phalanges. Among the Predentata the phalanges of the fifth toe are invariably absent in known forms, the formula, 2, 3, 4, 5, 0 being the usual one, and in Trachodon, 0, 3, 4, 5, 0. Among the