FINS AND LIMBS 



3i3 



developed. These girdles lie in the body-wall and are not 

 primitively connected with any other part of skeleton. 



In the bony fish, the scapula, coracoid and pelvis ossify 

 as cartilage-bones, and in addition, a number of dermal bones 

 arise in connexion with the pectoral girdle. In a primitive 

 bony fish like Polypterus, these dermal bones are the clavicle, 

 cleithrum, supra-clei thrum, and the post-temporal which is 

 attached to the hind part of the skull. This girdle, which is 

 composed of dermal bones, may be called the clavicular girdle, 



co 





e 



Fig. 158. — Comparison|between the fin of Sauripterus, A, and a pentadactyl 

 limb, B. (A after Gregory.) 



c, carpals ; co, coracoid ; h, humerus ; mc, metacarpals ; p, phalanges ; 

 r, radius ; ra, radials ; s, scapula ; u, ulna ; zo, web of the fin, supported by 

 lepidotrichia. The fin of Sauripterus is an example of the blunt paddle- 

 shaped type of fin. 



to distinguish it from the other girdle, formed of cartilage or 

 cartilage-bones, to which the term scapular girdle may be 

 applied. The clavicle is present in the sturgeon (Acipenser), but 

 in all higher fish it is lost. It may seem curious that the pelvic 

 girdle never has any additions of dermal bones to the cartilage- 

 bones of which it is composed. The explanation is that the 

 dermal pectoral girdle originally had no connexion with the 

 pectoral fins. It provides a firm attachment for the muscles 

 of the body-wall just behind the gill-slits in those forms (the 



