THE SKELETON IN REPTILES. POSTERIOR LIMB. 293 



is homologous with the post-pubis of Iguanodon. In Lacer- 

 tilia and sometimes in Chelonia there is a cartilaginous epipubis 

 attached to the anterior border of the pubic symphysis ; this is 

 well developed in the Chamaeleons and Geckos. In Crocodilia 

 there is, forming the anterior and ventral portion of the 

 acetabulum, a patch of cartilage (fig. 49, 3) which is probably 

 the true pubis homologous with that of lizards and with the 

 pre-pubis of Iguanodon. The large bone generally called the 

 pubis in Crocodiles is probably an epipubis. 



THE POSTERIOR LIMB. 



The posterior limb is entirely absent in some Lacertilia and 

 in most Ophidia, though traces occur in Python, Tortrix and 

 Typldops. In the Ichthyosauria, Sauropterygia and Pythono- 

 morpha the posterior limbs form swimming paddles and have 

 been already referred to. 



The arrangement of the proximal and middle segments of 

 the limb is fairly constant in all reptiles with limbs adapted 

 for walking, and the tibia and fibula are always separate. 

 The pes is however subject to a considerable amount of varia- 

 tion, especially as regards the tarsus. In some Chelonia the 

 tarsus like the carpus has an extremely simple arrangement, 

 consisting of a proximal row of three bones, the tibiale, inter- 

 medium and fibulare, a ceiitrale, and a distal row of five tar- 

 salia. In most living reptiles, however, the tibiale and inter- 

 medium are as in mammals united, forming the astragalus. 

 In Crocodiles (fig. 48, B, 15) the centrale is also united with 

 the tibiale while the distal tarsalia are very slightly developed. 

 The calcaneum in Crocodiles is drawn out into a long process 

 forming a heel in a manner almost unique among Sauropsida. 

 In Sphenodon and Lacertilia the tibia and fibula articulate 

 with a single large bone representing the whole proximal row 

 of tarsalia. 



The pes is generally pentedactylate, but in some Crocodiles 

 the fifth digit is vestigial (fig. 48, B), and in some Dinosauria 



