xiii PHYLUM CHORDA T A 225 



ventral position, as in Elasmobranchs, with the snout prolonged 

 over it. This is the case, for example, in the Sturgeons (Fig. 873) ; 

 in the allied Polyodon the snout takes the form of a horizon- 

 tally flattened shovel-like structure, about one-fourth the length of 

 the body. On the other hand, in the ground-feeding " Star-gazers " 

 and some other Acanthopteri 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 prolongation 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 (Fig. 874). Such a 

 projection is not to be confounded with the snout of the Sturgeon 

 or Polyodon, being formed by the elongation of the bones of the 

 jaws (premaxilla, maxilla, dentary, &c.), whereas in the two 

 Chondrostean forms referred to it is the anterior region of the 

 cranium which is prolonged. Still another form of "snout" 

 is produced in many Teleostei by the great mobility of the jaws, 

 allowing of their protrusion in the form of a short tube. In the 

 Wrasses or " lip-fishes" the mouth is bounded by fleshy lips 

 (Fig. 879, /p.). 



Tactile processes or barbels sometimes arise from the head ; the 

 most familiar example is that on the chin of the Cod and Haddock 

 (Figs. 873 and 877 &.). An operculum is always present, and is 

 supported by a variable number of investing bones; it is con- 

 tinued below into a branchiostegal membrane (Fig. 855, br. m.), 

 which, except in Crossopterygii and the Sturgeons, is supported 

 by bony rays. In Polypterus a pair of bony jugular plates (Fig. 

 872, B, jug. pi.) are placed at the lower end of the branchiostegal 

 membrane, between the rami of the mandible : Amia has a single 

 plate (Fig. 875, B, jug. pi.) in the same position. Spiracles are 

 present only in Polypterus (Fig. 887) and some Sturgeons. 



The commonest number of median fins is two dorsals, one caudal, 

 and one ventral, but this number may be increased or diminished 

 (Figs. 877 and 879), or there may be a continuous median fin ex- 

 tending along the back and round the end of the tail to the anus. 

 The dorsal fin is sometimes partly or wholly represented by a 

 series of small finlets (Fig. 872). The tail fin may be diphycercal, 

 heterocercal, or homocercal, and is usually the chief organ of 

 progression. But in the Sea-horse (Fig. 881) there is no caudal fin, 

 and the tail is prehensile, being used in the position of rest to coil, 

 in the vertical plane, round sea-weeds, &c. : when swimming it 

 hangs downwards, having no lateral movement, and locomotion is 

 effected by the vibration of the dorsal fin. 



The dermal-rays of the caudal fin are always jointed, as in 

 the Trout, but in most of the Acanthopteri and Pharyngognathi 

 more or fewer of the foremost rays of the dorsal, ventral and pelvic 

 fins are unjointed, forming spines (Figs. 878 and 879, d. /.)> some- 

 times large and strong enough to recall the dermal defences of 



