xii PHYLUM CHORDATA 393 



mouth sometimes has a ventral position, as in the Elasmo- 

 branchs, with the snout prolonged over it ; this is the case, 

 for example, in the Sturgeons. 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 

 produced by the elongation of the upper jaw, as in the 

 Sword-fish ; or of the lower jaw, as in the Half-beak or Gar- 

 fish ; or of both jaws as in the Bony Pike. 



An operculum or gill-cover (Fig. 219, 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 sup- 

 ported by four membrane bones. Ventrally the operculum is 

 produced into a thin membraneous extension, the branchio- 

 stegal membrane, which in nearly all cases is 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 supplanted 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 (Fig. 220). In some of the 

 Ganoids, however, this upward curvature of the caudal part 

 of the spinal column does not occur, and the tail is sym- 

 metrical internally as well as externally : in these the tail is 

 said to be diphycercaL In most Ganoids the tail is hetero- 

 cercal, as in nearly all the Elasmobranchs (p. 366). In some 

 Teleostomi the dorsal, caudal, and ventral fins form a con- 

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



