THE LIMBS 



197 



tionally didactyl, the extreme of specialization among reptiles. In 

 the Theropoda (Fig. 141 a-e) the thumb is the stoutest digit, its 

 claw the largest. In the herbivorous dinosaurs (Fig. 141 f, h, i, l, m) 

 the hand is less preaxial, the first and second fi.ngers being the larger. 

 In the Trachodontidae (Fig. 141 h), indeed, the first finger is ab- 

 sent. In all herbivorous forms the outer fingers are reduced, though 

 the fifth is seldom entirely absent, the phalangeal formula never ex- 

 ceeding 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 o, 3, 3, 2, 2. 

 The ungual phalanges of both front and hind feet are characteristic, 

 curved and sharply pointed in the Theropoda (Fig. 141 a-d), more 

 obtuse in the Sauropoda (Fig. 141 f, g), for the most part hoof- 

 like in the Predentata (Fig. 141 h-m). 



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 Cotylo- 

 sauria, 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 Dino- 

 sauria. The crocodiles (Fig. 157 a, b, d) have only the fifth meta- 

 tarsal 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. 1 54 c, d) ; 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, unhke most other 

 reptiles. 



The foot of dinosaurs (Fig. 156), so far as the reduction of pha- 

 langes is concerned, is less specialized than the hand, the Theropoda 

 (Fig. 156 a-e) retaining the original formula, except in the fifth toe. 

 Plateosaurus (a) and Anchisaurus (b), from the Trias, have the for- 

 mula 2,3,4, 5, 1 ; Allosaurus (c), from the lower Cretaceous, SindStru- 

 thiomimus (d), from the uppermost Cretaceous, 2, 3, 4, 5, o, the fifth 

 metatarsal a vestige. The known Sauropoda (i) have 2, 3, 4, 3, i 

 phalanges. Among the Predentata (f-h) the phalanges of the fifth 

 toe are invariably absent in known forms, the formula, 2, 3, 4, 5, o 

 being the usual one, and in Trachodon (g), o, 3, 4, 5, o. Among the 



