GANOIDEL 



321 



of each row being often articulated to those of the next by 

 distinct processes. In other cases the ganoid structures are 

 simply in the form of detached plates, tubercles, or spines; and 

 in some cases their shape is even undistinguishable from the 

 horny scales of the typical Teleostean fishes. In all cases, 

 however, whatever their form may be, they have the distinctive 

 ganoid structure, being composed of an inferior layer of true 

 bone and a superior layer of enamel. It is to be remembered, 

 however, that these ganoid plates and scales are not confined 

 to the fishes of the order Ganoidei, but that they occur in two 

 sub-orders of the Bony Fishes namely, the Plectognathi and 

 Lophobranehii and in some others of the Teleostei as well. 



III. As to the fins, both pectorals and ventrals are usually 

 present, and the. ventrals are always placed far back, in the 

 neighbourhood of the anus, and are never situated in the im- 

 mediate vicinity of the pectorals. In some living and many 

 extinct forms the fin-rays of the paired fins are arranged so as 

 to form a fringe round a central lobe (fig. 282). This struc- 



Fig. 282. Ganoid Fishes. A, Polypterus (recent) ; B, Osteolepis (extinct), a One 

 of the pectoral fins, showing the fin-rays arranged round a central lobe ; b One of the 

 ventral fins ; c Anal fin ; d Dorsal fin ; ct Second dorsal fin. 



ture characterises a division of Ganoids called by Huxley, for 

 this reason, Crossopterygidce, or "fringe-fmned." The form of 

 the caudal fin varies, the Ganoids being in this respect inter- 

 mediate between the Bony fishes, in which the tail is " homo- 

 cereal," and the Sharks and Rays, in which there is a " hetero- 

 cercal" caudal fin. In the majority of Ganoids, then, the tail 

 is unsymmetrical or " heterocercal," but it is sometimes equi- 

 lobed or " homocercal." 



