106 



CCJMPARATIVE ANATOMY 



previously put forward the view that the arches and fins correspond to meta- 

 morphosed gill-arches and rays : he supposed that one ray came to exceed 

 the others in size , and that the others then gradually became attached to it 

 instead of to the arch, the result bemg a biserial form of fin (" a/i-chiptery-, 

 (j'in)ii, "') which is most nearly retained in Ceratodus (Fig. 101 and p. 124). 



Pectoral Arch. 



Fishes and Dipnoans. — Paired fins and arches are wanting 

 in the Cyclostomi. In the Elasmobranchii and Holocephali the 

 pectoral arch consists of a comparatively simple cartilaginous bar 



Fig. 83. — Pectoral Arch and Fix of Hi-ptancliux. 



SB, SB^, pectoral arch, with a nerve aperture at XL ; Pr, J{s, Mt, the three 

 basal elements of the fin — pro-, meso-, and metapterygiimi ; Ra, cartilaginous 

 fin-rays ; a, b, the main fin-ray, lying in the axis of the metapterygium ; 

 +, single ray on the other side of the axis (indication of a biserial type) ; FS, 

 horny rays, cut through. 



the two halves of which are united ventrally by cartilage or fibrous 

 tissue (Fig. 83), and in embryos of Ganoids and Teleosts it has 

 at first a similar structure. 



Later, however, in both the last-named groups, a row of bony 

 structures arises in the perichondrium in this region ; so that a 

 secondary or bony pectoral arch may be distinguished from a 

 primary or cartilaginous one, the latter becoming less marked in 

 proportion to the development of the former (Fig. 84). 



The free extremity, or fin, is always connected with the hinder 



