222 



ZOOLOGY 



SECT. 



of thick, close-set, rhomboidal plates formed of bone, covered 

 externally by a layer of enamel-like material (ganoin) and joined 

 together by pegs and sockets. In many Ganoids the anterior 

 fin-rays of both median and paired fins bear a row of spine-like 

 scales called fulcra (Fig. 885, /.). True dermal teeth similar to 

 those of the Elasmobranchs occur scattered over the scales and 

 lepidotrichia in some Teleostomi (e.g. Lepidosteus, Polypterus) : 

 these may be fixed or movable (Siluroids). 



Endoskeleton. In the Sturgeons the vertebral column (Fig. 897, 

 WS.) consists of a persistent notochord with cartilaginous arches, 

 and is fused anteriorly with the cranium. In the remaining orders 

 bony vertebra3 are present ; the centra are biconcave, except 



in some Eels, 

 in which the 

 anterior face is 

 flat or even 

 convex, and in 

 Lepidosteus, in 

 which the an- 

 terior face is 

 distinctly con- 

 vex. Vertebra? 

 of this form, i.e. 

 having the cen- 

 trum convex in 

 front and con- 

 cave behind, 

 are called opis- 

 thoccelous. Ribs 

 are usually pre- 

 sent : in Poly- 



FIG. 896. Anterior end of vertebral column of Polypterus. PS. pterUS each V6r- 

 parasphenoid ; .R. / F, dorsal ribs: WK, centra ; t, ventral (pleura!) + >..., T,o a f w n 

 ribs. (From Wiedersheim's Comparative Anatomy.) 



pairs, a dorsal 



pair (Fig. 896, R, I 7) of considerable length, running between the 

 dorsal and ventral muscles, and a short ventral pair (f) between the 

 muscles and the peritoneum : the former answer to the ribs of Elasmo- 

 branchs, the latter to the ribs (pleural ribs) of the remaining Teleo- 

 stomi, which are always placed immediately beneath the peritoneum. 

 There may be one or more sets of intermuscular bones, attached 

 either to the neural arch (epineurals), to the centrum (epicentrals), 

 or to the ribs (epipleurals), not preformed in cartilage, but developed 

 as ossifications of the intermuscular septa. The posterior end 

 of the vertebral column is turned up in the Sturgeons, Lepidosteus, 

 and Amia, resulting in a heterocercal tail-fin : in Amia, however, 

 the fin-rays are so disposed that the fin appears almost symmetrical. 

 Among Teleostei the tail-fin is very usually homocercal, as in the 



