VII SCALES I 85 
In the remaining groups of Fishes, the Teleostomi and the 
Dipnoi, the spine of the primitive dermal denticle is either 
evanescent or entirely wanting, while the equivalent of the basal 
plate remains to form the unit of a scaly armature. Evidence 
of this may be found in the presence of transitory evanescent 
spines, provided with an enamel-cap, secreted by the basal epi- 
dermis, on the developing rhomboidal scales, as in the young 
Lepidosteus* (Fig. 101); while the entrance of blood-vessels into 
the scales through perforations on their inner surfaces, as in 
Polypterus and Lepidosteus, obviously recalls the perforated base 
of a dermal denticle (Fig. 99). The epidermis now ceases to 
Fic. 100, —Vertical section through the skin of an embryo Shark. C, Dermis; c¢.c.c.d, 
layers of the dermis ; #, epidermis ; e, enamel organ ; 0, enamel layer ; p, papilla of 
the dermis. (From Wiedersheim, after Gegenbaur.) 
take any part in the formation of the scales, and hence enamel 
no longer enters into their structure. A more regular and definite 
arrangement of the scales is noticeable, and whether distinct, or 
articulating with one another, or overlapping like the slates on 
the roof of a house, they are usually disposed in a series of 
successive oblique transverse rows. In some of these Fishes the 
embryonic epidermic covering of the scales becomes lost, and 
their outer surfaces are naked. More frequently, as in the 
generality of Teleosts, and in the Dipnoi, the reverse is the case, 
and the scales are more or less completely invested both by the 
dermis and the epidermis. As regards their shape, size, and 
minute structure there is much variation. In some Teleostomi 
1 Klaatsch, Morph. Jahrb. xvi. 1890, p. 125; Nickerson, Bull. Mus. Comp. Zool. 
Harvard, xxiv. 1893, p. 115. 
