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 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.
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.)
In Teleosts the usually thin and flexible scales are primarily developed from dermic papillae, but subsequently they come to lie in pockets or pouches in the dermis. As a rule no spines are developed, and so far no trace of an enamel organ has been detected during their development. The scales in their dermal pouches are disposed obliquely to the surface of the body, so that the hinder part of one scale overlaps the anterior portion of the scale next behind it (Fig. 104). Only the free hinder part of each scale has an epidermic investment (Fig. 105). In minute structure each scale consists of an outer layer of bone, which, like the bone of the endoskeleton, may either be homogeneous except for a feeble lamination, or it may contain bone-cells arranged in successive layers, parallel to the surface of the scale. In addition, there is an inner fibrous stratum in which the fibrous bundles in any one plane cross those in planes above or below them. The scales are either "cycloid," that is, they have smooth, unbroken margins (Fig. 105), or the free margin of each scale is produced into a series of tooth-like spines, and the scale is said to be "ctenoid" (Fig. 106, A).
Some Teleosts, however, have scales which are neither cycloid nor ctenoid, and in certain features seem to be intermediate between ordinary Teleostean scales and dermal denticles. Thus, on certain parts of the body of Centriscus,[[168]] each scale consists of a rhombic basal plate, produced into a curved, backwardly-inclined spine, the axis of which contains a pulp-cavity opening on the inner surface of the basal plate (Fig. 107). Some Malthidae (e.g. Malthe[[169]]) have similar scales, but with round basal plates and solid spines (Fig. 108, B). Similar scales (Fig. 109), sometimes rhombic in shape, with one or more spines, which may be simple or branched, are also found in the Sclerodermi (e.g. Balistes, Monacanthus, Triacanthus).[[170]]
Fig. 105.—Cycloid scale of Salmo fario. a, Anterior portion covered by overlap of preceding scales; b, free portion covered only by pigmented epidermis. (From Parker and Haswell.)
Fig. 106.—A, Ctenoid scale; B, "Ganoid" scale. (After Günther; from Parker and Haswell.)