cs 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 even 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 
Fic. 104.—Diagrammatic 
Sys=--— ep: longitudinal — section 
through the skin of a 
---8. Teleost to show the 
pam position of the scales. 
V1 d, Dermis; ep, epi- 
dermis; s, scale. 
(After Boas.) 
pit 
QL, 
a 
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- 
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,? each scale consists 
1 Klaatsch, op. cit. p. 178. 2 O. Hertwig, Morph. Jahrb. vii. p. 15. 
