12 REPTILES 



the specialised and comparatively modern group of Squamata 

 there is, however, a great tendency to the reduction of one or 

 both pairs of limbs, this tendency culminating in the snakes, 

 in all of which the limbs are altogether wanting, although re- 

 presented by slight internal and external vestiges in certain 

 families. Except in the more primitive types, the limbs of 

 reptiles exhibit great plasticity, so that in many cases they be- 

 come profoundly modified in accordance with the needs of 

 adaptation to special modes of life. 



A feature in the tarsus of all the members of the bird-like 

 brigade (Ornithomorpha) is that the ankle-joint occurs between 

 the upper and lower rows of small bones forming that segment 

 of the skeleton, and not, as in mammals, between the upper row 

 of the tarsus and the lower ends of the tibia and fibula. But 

 this is by no means all, for in certain dinosaurs which habitually 

 assumed the upright posture the astragalus and calcaneum, 

 forming the upper part of the tarsus, become closely applied 

 to the tibia and fibula so as to form practically a single bone 

 with each. Moreover, the tibia, with the closely applied astra- 

 galus, may become the sole functional bone of this segment of 

 the limb. In the figured specimen the astragalus sends up only 

 a short process in front of the tibia, but in other instances, as in 

 Dryptosaitnis, this process becomes much larger and is in fact 

 practically similar to that of the astragalus of birds, which in the 

 adult condition becomes indissolubly fused with the tibia. 

 Furthermore, in certain dinosaurs with three toes the three sup- 

 porting metatarsal bones become closely applied to one another 

 and likewise to the lower row of tarsal bones (reduced to two in 

 number), thus combining, at all events occasionally (? patho- 

 logically) to form a cannon-bone corresponding to the tarso- 

 metatarsus of birds. And yet even this extreme instance is but 

 a case of parallel adaptive modification for the same end 

 (namely, the assumption of the upright posture), for it seems 

 quite evident that these dinosaurs were not the ancestors of 

 birds. Nevertheless other reptiles with a tibio-tarsus and a 

 tarso-metatarsus, or, what is the same thing, birds with a 

 separate tarsus and disunited metatarsals, must once have 

 existed. 



Whether in the tarsus of the mammal-like brigade the ankle- 

 joint normally occurred between the upper and the lower rows 



