SCALES 



I8S 



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 Zepidosteus, obviously recalls the perforated base 

 of a dermal denticle (Fig. 99). The epidermis now ceases to 



Fia. 100. — Vertical section through the skin of an embryo Shark. C, Dermis ; c.c.c.d, 

 layers of the dermis ; U, epidermis ; e, enamel organ ; o, 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 



^ Klaatsch, Morph. Jahrb. xvi, 1890, p. 125 ; Nickerson, Bull. Mus. Comp. Zool. 

 Harvard, xxiv. 1893, p. 115. 



