TELEOSTOMOUS FISHES. 243 



The gill slits do not open directly to the external world, but 

 into a gill chamber formed by an operculum or fold, which 

 extends backwards on either side from the hyoid arch ; and this 

 operculum is strengthened by bone. The body may be naked 

 or scaled, but placoid scales never occur, and claspers are never 

 developed in connection with the pelvic fins. 



The most marked characters are presented by the skeleton. 

 In this the cartilage may be partially or almost completely 

 replaced by bone, and besides, there is always an extensive 

 formation of membrane bones, unknown in the recent members 

 of the other groups of fishes. In all except the sturgeons the 

 vertebral centra are ossified, and in all except the sturgeons and 

 the garpikes the vertebrae are amphiccelous. More or less 



SOr 



FIG. 245. Skull of pike, Esox lucius, from Huxley. An, angulare; Av, 

 articulare ; Brg, branchiostegals ; D, dentary ; HM, hyomandibular ; IOp, inter- 

 opercle; Alt, metapterygoid ; MX, maxillary; Op, operculum; PI, palatine; Prnx t 

 premaxilla; Prf, prefrontal ; PrOp, preoperculum ; Qu, quadrate; SOp, subopercle; 

 SOr, suborbital ; Sy, symplectic. 



extensive ossifications occur in the chondrocranium, and besides, 

 a large number of dermal bones are developed, which roof in 

 the cranium above and build it out in other places. Excepting 

 the dipnoi, the cranial structures of which will be described 

 later, the most constant and most characteristic of these bones 

 are the following : The upper jaw is formed by a pair each of 

 maxillaries and premaxillaries, while the roof of the mouth is 

 formed by a pair each of vomers and palatines and a parasphe- 

 noid, all of which may bear teeth. Thus the pterygoquadrate 

 no longer forms the upper jaw as in the lower groups, but be- 

 comes deeper in position, and undergoes more or less extensive 

 ossification, sometimes developing but two bones, pterygoid 



