. The Cambridge natural history. Zoology. FISHES CHAP. 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) ^ 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 l)ody is traversed by five widely-separated longitudinal rows of large
. The Cambridge natural history. Zoology. FISHES CHAP. 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) ^ 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 l)ody is traversed by five widely-separated longitudinal rows of large bony scutes, which, like the rhombic scales, are furnished with ridges and projecting. Fig. 103.—Surface view of the rhombic scales of a youug 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 ; , intersecting fibrous bands of the dermis ; 5, vestigial spines. (After Klaatsch.) spines (Fig. 102). Between the rows of large scales there are numerous denticle-like structures aiTanged 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 Polyoclon 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. Jahri. ii. 1876, p. 374 ; Klaatsch, xvi. 1890, p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 18
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
