188 THE VERTEBRATE SKELETON. 



pubis. In Xenopus, however, the bone corresponding to the 

 ilium of the Frog is seen to ossify from two centres, one 

 forming the ilium, the other, which lies at the symphysis, 

 being apparently the pubis. This makes it probable that the 

 so-called ilium of the Frog is really to be regarded as an ilio- 

 pubis, and renders the homology of the cartilaginous part 

 uncertain, but it probably corresponds to the acetabular bone 

 of mammals. In Xenopus also there is a minute epipubis 

 similar to that of Urodeles. 



POSTERIOR LIMB. 



In Urodela the posterior limb (fig. 31, B) closely resembles 

 the anterior limb, but is even less removed from the primitive 

 condition of the higher vertebrates in the fact that all five digits 

 are commonly present. The tibia and fibula are short bones 

 approximately equal in size. In some cases the number of 

 digits is reduced. Thus in Menobranchus the pes has four 

 digits, in Proteus it has two, and in Amphiuma two or three, 

 while in Siren the posterior limbs have atrophied. 



In correlation with their habits of jumping, the posterior 

 limbs in Anura are much lengthened and considerably modified. 

 The tibia and fibula are completely fused. The intermedium 

 is absent, while the tibiale and fibulare are greatly elongated. 

 Tarsalia 4 and 5 are absent. Five digits are always present, 

 and there is a pre-hallux formed of two or more segments. 



In general the posterior limbs in Labyrinthodontia bear the 

 closest resemblance to the anterior limbs ; in some cases three 

 centralia are found. 



In Ichthyoidea, and in most Labyrinthodontia, the carti- 

 lages of the carpus and tarsus remain unossified; in Salaman- 

 drina and in Anura they are generally ossified. 



