LIMBS 



127 



distal are called carpalia or tarsalia (in the narrower sense). They 

 are counted beginning from the pre-axial (radial or tibial) side of 

 the limb. 



Amphibia. — The anterior and posterior extremities of Urodela 

 are formed essentially on the ground-plan described above. There 

 are usually five digits in the hind-limb, and always four in the 

 fore-limb. In the Anura the radius and ulna become fused together, 

 and a separate intermedium is wanting ; the proximal row of the 

 tarsus, moreover, consists of only two cylindrical bones, one of which 

 {astragalus) corresponds to a tibiaie, and the other (calcaneum) 

 to a fibulare.^ 



In the distal row of the carpus four separate elements are 

 formed, but this number may become reduced owing to secondary 

 fusions ; in rare cases a fifth carpal may 

 also be present. Very different views 

 exist as regards the homologies of the 

 individual carpals of Anurans. In the 

 distal row of the tarsus, tarsalia II and 

 /// are the most constant elements, 

 but even these may undergo fusion ; 

 tarsalia IV and V are generally repre- 

 sented by a ligament ; and tarsale I 

 usually does not long remain distinct. 



In Anura the metatarsals and 

 phalanges, between which the web of 

 the foot is stretched, are very long and 

 slender. The femur, as well as the 

 bones of the shank, which are fused 

 together, are also exceedingly long, in 

 correspondence with the mode of pro- 

 gression of these animals. The skeleton 



D 



of the extremities is more strongly 

 ossified in Anurans than in Urodeles, 

 in which many of the elements remain 

 cartilaginous. 



Traces of an extra toe {i^rehalhtx) 

 occur on the tibial side of the tarsus, and in both Urodeles and 

 Anurans indications of an additional pre-axial digit in the manus 

 are occasionally met with. The number of phalanges on the 

 individual digits varies in different Amphibians. 



Rudiments of the extremities can be recognised externally in embryos of 

 the limbless Gymnophiona. 



Reptiles. — Chelonians and Lizards (and more especially Hal- 



FiG. 106. — Skeleton of pthe 

 Shank, Tarsus, ASDiFooT 

 OF Speltrpes fuscus. 



tb, tibia ; fb, fibula ; 1, tibiaie ; 

 !, intermedium ; /, fibu- 

 lare ; c, centrale ; 1-5, 

 tarsalia ; i-i\ digits. 



1 It is possible that the tibiaie and fibulare also include the representatives of 

 other elements. 



