BONE 



355 



structure (Figs. 340, 341). This is brought about by the reduction 

 of the extreme tip of the heterocercal or homocercal fin in the later 

 stages of development ; such a false or secondary diphycercal fin is 

 called gephyrocercal (Ryder [378]). The structure of the actual 

 upturned tip of the notochord varies greatly, being naked in Eso?, 

 with a cartilaginous sheath in Salmo, Slops, etc., a bony urostyle in 



FIG. 340. 



Skeleton of the extremity of the tail of Fierasfer dentatus, Cuv. (After Emery.) 

 /, lepidotrich ; v, last vertebra. 



Acanthopterygii and others. Special bony plates may lie on each 

 side of it (Fig. 63) (Lotz [286]). 



It is to be noticed that although the caudal fin is chiefly of 

 hypochordal origin (p. 104), yet a considerable portion of the upper 

 lobe may be derived from the epichordal fin (Fig. 46). The 

 composition of the caudal fin thus varies in different families, and 

 ii more exact study of its development might yield useful results. 



FIG. 341. 



Callionymm lyra, L. Left-side view of the two last caudal vertebrae, enlarged, a.p, anterior 

 articulating process ; c, centrum ; h, hypural expansion ; t, outline of tail. 



In a large number of the more primitive Teleostei the bone in 

 the adult is of normal structure with branching bone-cells, vascular 

 canals, and a lamellated matrix (p. 61) ; but in many others it 

 becomes strangely modified (Kb'lliker [270], Schmidt-Monard [388], 

 Stewart [425]). For instance, in Salmo and Thymallus the cells lose 

 their branching processes ; in Xiphias gladius the lamellated matrix 

 is deposited round vascular canals some of which give off fine 

 tubules ; but the bone-cells are very scarce or altogether absent. 

 Fistulariii and the Plenronectidae have likewise lost the cells and 



