In the surviving Crossopterygii, as in _Polypterus_, the scales are rhomboidal and thick, and they only slightly overlap. They articulate with one another by means of marginal peg-and-socket articulations (Fig. 106, B). A thick layer of hard, glistening, enamel-like substance or "ganoin" forms the outer layer of the scale; the inner layer consisting of bone in which dentinal tubules as well as bone-cells are present. In the numerous fossil members of the group the scales are either rhomboidal or cycloid. {188}The oldest representatives of the Chondrostei, the Palaeoniscidae (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_)[166] the primitive rhombic squamation is retained only on the sides of the terminal part of the tail, and there they are in close apposition 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 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. [Illustration: 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.)] Among the Holostei the scales are very different in the two surviving members of the group. In _Lepidosteus_ (Fig. 103) the {189}scales are rhombic, and both in arrangement and structure, as well as in their method of articulating with one another, they closely resemble those of _Polypterus_. In _Amia_,[167] on the contrary, the relatively thin scales are cycloid in shape, and in their imbricated arrangement, in their enclosure in pouches of the dermis, and in the absence of any superficial covering of ganoin, they are very similar to the scales of the more typical Teleosts (Fig. 295). The resemblance extends even to histological structure, for each scale consists of an outer layer of bone, which gradually passes into an inner fibrous stratum. [Illustration: FIG. 104.—Diagrammatic longitudinal section through the skin of a Teleost to show the position of the scales. _d_, Dermis; _ep_, epidermis; _s_, scale. (After Boas.)]