28o 



VERTEBRATE SKELETON 



The femur, only its shaft ossified, is long and has both tibial and 

 fibular trochanters, one on either side of the head. Tibia and fibula 

 are subequal in length and are separate. The foot, with few excep- 

 tions {Proteus down to two) has five digits and the tarsal parts are 

 typical, except that two or three of the tarsaha may fuse, while 

 occasionally there are two or three centraha. 



Anura have the humerus straight in many genera, but curved in 

 many toads. It is longer than the forearm and usually has a single 

 tuberosity. Ulna and radius are fused, but retain two articular 

 surfaces at the distal end. Rarely {Rana, Pipa) a branchial patella 

 occurs. The carpus is ossified, but the homo- 

 logies of the seven ossicles of the adult are not 

 certain, although the ontogeny is partly known. 

 Besides the normal elements, one or two small 

 bones may occur on the radial side often regarded 

 as pollex and prepollex. 



The saltatory functions of the hind hmbs 

 are correlated with structure, especially with 

 the elongation of femoral, crural and tarsal 

 regions. The femoral trochanters are weak or 

 obsolete. Tibia and fibula are united, the line 

 between them being most evident in Pipa, least 

 so in Rana. No intermedium appears in 

 ontogeny, there being two long bones — tibiale 

 (talus, astragalus) and fibulare (calcaneum) 

 in the proximal tarsal row. These are separate in Bufo and 

 Bombinator, fused at both ends in Rana (fig. 303), throughout in 

 Alyles. The tarsaha are reduced to from one to three small bones 

 at the base of the first three metatarsals. No centrale is known, a 

 fourth tarsale is said to fuse with the fibulare. The elements on the 

 tibial side are often interpreted as prehallux (fig. 303). 



REPTILIA. — Most reptiles have the body hfted slightly above 

 the ground, the marked exceptions being the aquatic Ichthyosaurs, 

 Sauropterygians, Thalattosaurs, some Chelonians and Pythono- 

 morphs; and the flying Pterosaurs. Many Dinosaurs had bipedal 

 locomotion. An aquatic hfe tends to shorten the stylopodial and 

 zeugopodial bones, transforming the legs into paddles, this being 

 accompanied by an elongation of the digits, either by lengthening the 



Fig. 303. — Tarsus of 

 Rana. c, centrale; /, 

 fibulare; m, metatarsals; 

 ph, prehallux; t, tibiale; 

 1-3, tarsalia. 



