THE LIMBS 20I 



toes have become shortened and the phalanges on the postaxial side 

 reduced, the dinosaurs have rather long digits, but they had become 

 distinctively digitigrade, shortening the portion resting upon the 

 ground, like the reduction of the digital formula in the plantigrade 

 reptiles. In all such reptiles with the more mammal-like mode of 

 locomotion, the foot is more mesaxial or preaxial, as in the mam- 

 mals, where the fourth is very seldom the strongest toe. 



The chief joint between the foot and legs in mammals is between 

 the end of the tibia and the first row of the tarsals. In reptiles it is 

 intratarsal, that is, between the first and second rows of the tarsus. 

 In those reptiles which walked more or less upon the toes, digitigrade, 

 there was a progressively closer articulation between the tibia and 

 the astragalus, giving a firmer and closer ankle which otherwise 

 would have been subject to injury with the heel elevated far above 

 the ground. In the bipedal Theropoda (Fig. 156 a, c), the astrag- 

 alus, while perhaps never fully fused with the tibia, acquired a long 

 ascending process which fitted closely into a groove in front of the 

 distal end of the tibia. In the still more elongated feet of the ptero- 

 dactyls (Fig. 155 c, d) the astragalus became indistinguishably fused 

 with the tibia, as in birds, and the joint, while actually, as formerly, 

 intratarsal, was functionally between the leg and tarsus as in mam- 

 mals. 



Short toes and reduction of phalanges, then, mean a more mam- 

 mal-like mode of locomotion and posture of the feet. In the turtles 

 this has been produced by the exigency of the immovable shell, and 

 by the greater or lesser twisting of the epipodials upon the pro- 

 podials. 



