6o 



The Skeleton of the Fish 



In the Ichtliyotomous sharks, Dipnoans, and Crossoptery- 

 gians the segments of the pectoral limb are arranged axially, 

 or one beyond another. This type of fin has been called 

 arcJii pterygium by Gegenbaur, on the theory that it represents 

 the condition shown on the first appearance of the pectoral fin. 

 This theory is now seriously questioned, but it will be convenient 

 to retain the name for the pectoral fin with segmented axis 

 fringed on one or both sides by soft rays. 



Fig. 50. — Shoulder-girdle of a Hake, Merluccius produrtu.'i (Ayres). 



The archipterygium of the Dipnoan genus Xeoceratodns is 

 thus described by Dr. Gunther (" Guide to the Study of Fishes," 

 p. 73) : " The pectoral hmb is covered with small scales along the 

 middle from the root to the extremity, and is surrounded by a 

 rayed fringe similar to the rays of the vertical fins. A muscle 

 s[)lit into numerous fascicles extends all the length of the fin, 

 which is flexible in every part and in every direction. The 

 cartilaginous framework supporting it is joined to the scapular 

 arch by a broad basal cartilage, generally single, sometimes 



