82 



ZOOLOGY 



SECT. 



ends of which are articulated with stout cartilages (Has), often 

 replaced by bones, the basalia, which serve to strengthen the fin at 

 its point of union with the trunk. 



In all classes above Fishes the paired fins are, as we have seen, 

 replaced by five-toed or pentadactyle limbs. These are supported by 

 bones, probably to be looked upon as greatly modified pterygiophores, 



FIG. 700. Diagram of three stages in the development of the pelvic fins. In A the anterior 

 pterygiophores on the right side (Had), have united to form a basal cartilage (Bas.) ; in B the 

 basalia (Bas.) are fully formed and are uniting at * to form the pelvic girdle ; in C the pelvic 

 girdle (G) is fully constituted, and at t has segmented from the basale on the right side. 

 CL. cloacal aperture. (From Wiedersheim's Comparative Anatomy.) 



and obviously homologous in the fore- and hind-limbs. In the proxi- 

 mal division of each limb there is a single rod-like bone, the humcrus 

 (Fig. 767, HU), or upper-arm-bone, in the fore-limb, the femur 

 (Fig. 768, FE,) or thigh-bone, in the hind-limb. In the middle 

 division there are two elongated bones, an anterior, the radius 

 (RA), and a posterior, the ulna (UL), in the fore-limb ; an anterior, 

 the tibia (TI), and a posterior, the fibula (FI), in the hind-limb. 

 Next follow the bones of the hand and foot, which are again 

 divisible into three sets : carpals or wrist-bones, nietacarpals 



