. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. TELEOSrOMOUS 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, th
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. TELEOSrOMOUS 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 amphicoelous. More or less SOfr. Ar An SyPrb/i Srg 1^ Fig. 245. Skull of pike, Esox lucius, from Huxley. An, angulare; Ar, articulare; Brg, branchiostegals; D, dentary; HM, hyomandibular; lOp, inter- opercle; Ml, metapterygoid; Mx, maxillary; Op, operculum; PI, palatine ; Piiix, 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. Plea
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