478 - 



COIMPAEATIVE ANATOIMY. 



lateral rays are considerably developed. The remains of tlie medial 



row are ordinarily quite small (Fig. 263, B'), but they are always 



sufficiently distinct to justify us in sup- 

 posing that in higher forms the two sets 

 of rays might be better developed. Rays 

 are still attached to the stem, and are 

 connected with the shoulder-girdle by 

 means of larger plates (p ms). The 

 joints of the rays are sometimes broken 

 up into polygonal plates^ which may, 

 further, fuse with one another ; concres- 

 cence of this kind may also affect the 

 pieces which form the base of the fin 

 {jy ms). By regarding the free rays, 

 which are attached to these basal pieces, 

 as belonging to these basal portions, we 

 are able to divide the entire skeleton of 

 the fin into three segments — pro-, meso-, 

 and metapterygium. 



The metapterygium (mt) represents 

 the stem of the archipterygium and the 

 rays on it. The propterygium (p) and 

 the mesopterygium (ms) are evidently 

 derived from rays which still remain 

 attached to the shoulder-girdle. 



The peculiar form of the fin in the 

 Ray is due to the great development of 

 the propterygium ; the arrangement in 

 Squatina leads towards this. One ray 

 is here converted into a support for 

 rays, and forms, by gradually reaching 

 forwards, a stem for the propterygium, 

 just as the metapterygium in the stem 



of the archipterygium possesses one. The Chimeeree agree in all 



essential points with the Sharks. 



Fig. 263. Skeleton of the 

 thoracic fin of Acanthias 

 vulgaris, p Basale of the 

 protopterygimn. mt Of the 

 metapterygium. B Median 

 edge of the fin. The lino 

 drawn through mt indicates the 

 series which formed the stem 

 of the archipterygium. The 

 dotted lines correspond to the 

 rays, which are mostly arranged 

 at the sides (R R), and are 

 rudimentary only on the medial 

 Bide (R'). 



§ 362. 



The skeleton of the thoracic fin in the Ganoidei may be derived 

 from a condition which is similar to that which obtains in the Shark ; 

 it is the same fin, with the peripheral parts reduced (cf. Fig. 264). 

 In correspondence with this, a few rays only are attached to the 

 stem of the fin (B), and those which are set on the shoulder-girdle 

 are also rudimentary. In the Teleostei the peripheral portion of the 

 skeleton of the fin is still further reduced, and as a rule nothing 

 remains of the primary fin-skeleton except four or five elements 

 which are very similar to one another (Fig. 261, g) ; a very variable 

 number of small and always cartilaginous pieces are attached 

 peripherally to them. These, then, serve as supports for the 



