992 



CHAPTER L. 



CLA SS PIS CE S — continued. 



Order Teleostei. 



WAV ; 



Order VI. Teleostei. — The last, and in many respects the 

 most highly organised, order of Fishes is the Teleostei, which in- 

 cludes the greater number of existing forms, and (unless some of 

 the genera here placed in the Amioidea belong to it) does not date 

 back beyond the Cretaceous. The Teleostei are in all probability 

 descended from the Ganoids, and occupy in the class a somewhat 

 analogous position to that held by the Squamata among the Reptiles 

 and the Passeres among Birds ; all traces of Amphibian affinities 

 having been entirely lost in this order. 



It is impossible to give a definition of the order by which it can 

 be sharply separated from the Ganoids, but the following are its 



most characteristic fea- 

 tures from a palseon- 

 tological point of view. 

 The body is usually 

 covered with thin elas- 

 tic, cycloid, or ctenoid 

 scales (figs. 832, S33); 

 but bony scutes or gan- 

 oid scales are occasion- 

 ally present. The whole 

 of the endoskeleton is 

 well ossified ; and the 

 gills are freely suspend- 

 ed in a gill-cavity cover- 

 ed by a well-developed 

 operculum (fig. 930). 

 The caudal fin of the 

 adult is of that completely masked heterocercal type usually termed 

 homocercal (fig. 931). The pelvic fins may be either abdominal 



Una?-.. 



M: 



My 



Fig. 930. — Skull of Trout (Salttto). Reduced, 

 as in fig. 843, p. 917. 



Letters 



