188 FISHES CHAP. 
The oldest representatives of the Chondrostei, the Palaeonis- 
cidae (Fig. 285) 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)' 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 
Fic. 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 ; 
7.6, 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 O. Hertwig, Morph. Jahrb. ii. 1876, p. 374; Klaatsch, xvi. 1890, p. 146. 
