iSS 



FISHES 



The oldest representatives of the Chondrostei, the Palaeonis- 

 cidae (Fig. 283) possessed a complete armature of rhombic 

 scales, but in all the surviving members of the group the scales 

 have undergone considerable modification in some respects, and 

 in others are degenerate. In the Sturgeon (Acipenser*) l the 

 primitive rhombic squamation is retained only on the sides of 

 the terminal part of the tail, and there they are in close apposi- 

 tion in oblique rows. The rest of the body is traversed by five 

 widely-separated longitudinal rows of large bony scutes, which, 

 like the rhombic scales, are furnished with ridges and projecting 



c. 



-fb. 



FIG. 103. Surface view of 

 the rhombic scales of 

 a young Lepidosteus. 

 In two scales the parts 

 which are overlapped 

 by adjacent scales are 

 shaded, c, Position of 

 the central canal which 

 perforates the inner 

 surface of each scale ; 

 f.b, intersecting fibrous 

 bands of the dermis ; s, 

 vestigial spines. (After 

 Klaatsch.) 



spines (Fig. 102). Between the rows of large scales there are 

 numerous denticle-like structures arranged in oblique rows. Each 

 of these consists of a basal plate imbedded in the dermis, and of 

 one or more projecting spines which perforate the epidermis. 

 All the scales have the same minute structure, consisting mainly 

 of bone ; but the surface layer and the spines seem to be composed 

 of a hard laminated substance from which bone-cells are absent 

 (ganoin). In Polyodon the scutes are wanting, but vestigial 

 denticles are retained. 



Among the Holostei the scales are very different in the two 

 surviving members of the group. In Lepidosteus (Fig. 103) the 



1 0. Hertwig, Morph. Jahrb. ii. 1876, p. 374 ; Klaatsch, xvi. 1890, p. 146. 



