314 COMPARATIVE ZOOLOGY OF CHORD ATES 



bony fish) in which the gill-slits are highly developed. The 

 gill-slits occupy a region of perforation and weakness, and they 

 prevent the main mass of the lateral body-wall muscles from 

 becoming attached to the skull. The dermal pectoral girdle, 

 which itself is attached to the skull, gives these muscles some- 

 thing solid to work from. The joining of the scapular and 

 clavicular pectoral girdles is due to the fact that both are 

 situated close behind the gill-slits. Since there are no gill- 



Fig. 159. — The forelimb of Sphenodon : an example of a typical penta- 

 dactyl limb with a primitive carpus. 



The figures indicate the ordinal numbers of the digits, c, carpals of the 

 distal row, which are five in number ; ce, centralia ; h, humerus ; i, inter- 

 medium ; mc, metacarpal ; p, phalanges ; ps, pisiform ; r, radius ; ra, 

 radiale ; w, ulna : id, ulnare. 



slits or other source of weakness near the pelvic girdle, the 

 latter has no dermal elements added to it. 



From the nature of the water in which they live, the fins 

 of fish are necessarily more or less like paddles. But it is from 

 such paddles (or ichthyopterygia) that the five- digi ted or penta- 

 dactyl limb (cheiropterygium) of the Tetrapods or land- 

 vertebrates was evolved. It is interesting to inquire into the 

 question as to which type of fin most probably gave rise to the 



