396 MANUAL OF ZOOLOGY SECT. 



for example, in the sturgeons (Fig. 240). On the other 

 hand, in the ground-feeding " star-gazers " and some others, 

 the lower jaw is underhung like that of a bull-dog, and the 

 mouth becomes dorsal in position. A beak may be pro- 

 duced by the elongation of the upper jaw, as in the sword- 

 fish, or of the lower jaw, as in the half-beak or czar-fish, or 

 of both jaws as in the bony pike. 



An operculum or gill-cover (op), a flap which covers the 

 gills of each side and bounds in front the single, usually 

 crescentic gill-opening, is always present, and is supported 

 by four membrane bones. Ventrally the operculum is pro- 

 duced into a thin membranous extension, the branchio- 

 stegal membrane, which is in nearly all cases supported by a 

 series of bony rays. Spiracles are absent except in certain 

 of the Ganoids. 



There are dorsal, ventral, and caudal median fins. The 

 dorsal is usually divided into two ; in a few it is partly or 

 wholly supported by a series of finlets. The caudal is in 

 the majority of a type to which the term homocercal is 

 applied. The homocercal caudal fin is divided into two 

 equal or sub-equal lobes, upper and lower, so that it appears 

 symmetrical externally, though the posterior portion of the 

 spinal column which supports it is strongly bent upwards 

 and terminates in the upper lobe. In some of the Ganoids, 

 however, this upward curvation of the caudal part of the 

 spinal column does not occur, and the tail is symmetrical 

 internally as well as externally ; in these the tail is said to 

 be diphycercal. In many Ganoids the tail is heterocercal, as 

 in nearly all the Elasmobranchs (p. 367). In some Teleos- 

 tomi dorsal, caudal, and ventral fins are united into a con- 

 tinuous fold. The dermal fin-rays of the caudal fin and a 

 portion or all of those of the rest of the fins are slender 

 flexible rods divided into a series of short segments and 



