VII SCALES 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} on the contrary, 

 the relatively thin scales are cycloid in shape, and in their im- 

 bricated 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 e.ven to histological structure, 

 for each scale consists of an outer layer of bone, which gradually 

 passes into an inner fibrous stratum. 



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 



Pig. 104. — Diagrammatic 

 _ gTj. longitudinal section 



through the skin of a 

 -S. Teleost to show the 



-d. position of the scales. 

 d, Dermis ; ep, epi- 

 dermis ; s, scale. 

 (After Boas.) 



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 homo- 

 geneous except for a feeble lamination, or it may contain bone- 

 ceUs 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 Gentriscus^ each scale consists 



1 Klaatsch, op. cit. p. 178. ^ 0. Hertwig, Morph. Jahrh. vii. p. 15. 



