DEVELOPMENT OP THE SCALES AND BONES OF FISHES. 
651 
On directing our attention to the uppermost layer, 9 c, we find a considerable dif- 
ference presenting itself. When the scales of the Carp are in situ, the free portion of 
each, or that which is not covered over by its antecedent neighbours, is very rough, 
owing to the existence of numerous tooth-like points which project from its surface. 
It is this portion of the scale that has been traversed by the section fig. 9, and from 
it we learn that the existence of the superficial teeth is the result of an extraordinary 
development of the uppermost layer, which has not only produced these projecting 
appendages, but has also thickened the intervening portions. Nothing can be more 
distinct than the structures of the central and superficial tissues, as they exist in this 
scale. The superimposed lamellm, of which the latter of these consists, are most ob- 
vious, even under a low magnifying power ; and on tracing their direction, we see that 
it corresponds with the superficial outline of the section, evidently showing that its 
increment is effected at the upper surface. This appears to have been accomplished 
by the calcification of a very thin membrane with which the scale is covered, at the 
same time that the corresponding, though different, process was making additions to 
the lower surface of the middle layer along the undulating line, 9 d. The structure of 
these two portions is as distinct as their modes of growth. The middle tissue (9 Z») 
exhibits the rough laminae, filled with vertical fissures and traces of the lenticular 
granules of which it consists. The lamellae of the uppermost one, on the other hand, 
appear to be perfectly structureless ; consisting, apparently, of a smooth and homo- 
geneous tissue, not to be distinguished, in the vertical section, from that seen in the 
ganoin of a Lepidosteiis or some of the Lepidoti ; thus the differences of aspect and 
of growth alike bring us to the conclusion that the scale of the Carp consists of three 
very distinct structures. At the anterior portion of the same vertical section, we find 
that the middle layer disappears as in the preceding scale, whilst the upper one rests 
immediately upon the membranous lamellae. Here, though very thin when compared 
with its more largely developed posterior portion, the superior layer exhibits nume- 
rous minute tooth-like projections, which, whilst they exhibit the same laminated 
structure as the larger ones previously noticed, are more uniform in their size as well 
as more regular in their distribution. These projections are produced by the vertical 
and transverse section of the numerous concentric lines which give the cycloid aspect 
to this portion of the scale. It thus becomes manifest that these concentric ridges 
are not lines of growth, as thought by M. Mandl, but the result of a peculiar ar- 
rangement of the superficial tissue of the scale, a conclusion which accords with that 
arrived at by M. Agassiz. 
The scales of the Pike, Salmon, and numerous other allied fish, are constructed on 
a similar plan to those already described, varying only in their minor details. A re- 
markably fine illustration of the type requiring a more special notice, is that of the 
Perch {Perea jiuviatilis, Linn.), which also illustrates the peculiarities of the Ctenoid 
scale. In a vertical section of one of these, we observe that the number of the small 
lenticular granules which are isolated and dispersed through the membranous lamellce. 
