488 



COMPARATIVE ANATOMY. 



arc attached to the leg; these are the fibulare, intermedium, and 



tibiale. There are two centralia ; and five distal tarsalia carry the 



bones of the metatarsus (cf. Fig. 265). 



In the Enaliosaurii the skeletal portions of the hinder extremity 



are an exact repetition of those of the anterior one ; and even in 



some of the Amphibia (Urodela) we meet 

 with an arrangement which is the same 

 in all essential points, so that we need 

 not describe them specially. In most 

 Urodela, all the five terminal pieces, or 

 toes, are retained in the hind-limb ; this 

 is more distinctly like the primitive form 

 than is the skeleton of the fore-limb. 

 In Cry ptobran elms, Menopoma, and 

 others, the two centralia even are per- 

 sistent. But in the Anura there is a 

 very great change ; the tibia and fibula 

 are fused. In the place of the three 

 proximal tarsal bones there are two 

 long bones, which are, however, often 

 fused at their ends ; they are ordinarily 

 known as the astragalus and calcaneum. 

 The distal row of tarsal bones is also 

 greatly reduced. Finally, we must note 

 the presence of a rudiment of a sixth toe. 



§ 368. 



In the Chelonii there are unimportant 

 modifications in the larger pieces of the 

 extremities ; in addition to this we must 

 note the gradual concrescence of some 

 of the bones of the tarsus, which is of 

 great importance as explaining the ske- 

 leton of the foot in Birds, as well as in 

 other Reptiles. An intermedium is united 

 with a tibiale to form an astragalus ; and 

 the centrale is attached to, or even com- 

 pletely fused with, this bone. The fourth 

 and fifth tarsalia similarly form a single 

 bone, the cuboid. Owing to the forma- 

 tion of a single piece out of the bones 

 of the first tarsal row, and the firm union that is effected between 

 this piece, and the tibia and fibula, the foot gets to be articulated 

 in a peculiar manner. It moves on an intertarsal joint. The 

 skeleton of the Crocodile's foot is somewhat different. The tibia 

 and fibula articulate with two bones, of which the fibulare has the 

 greater power of movement. The larger bone, connected with the 

 tibia, corresponds to the similar bone in the Chelonii. A piece of 



Fig. 273. Hinder extremity of a 

 larva of Salamandra macu- 

 losa. The dotted lines arc 

 drawn through the rays, to 

 which the different pieces he- 

 loner. 



