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 Esox, 

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



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

 1, lepidotrich ; u, 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 

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



Callionymiis 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 (Kolliker [270], Schmidt-Monard [388tt], 

 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. 

 Fistularia and the Pleuronectidae have likewise lost the cells and 



