DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
655 
Lepidoid Fishes ? I confess I see no way of distinguishing it from the true ganoin 
itself, using the term in the restricted sense in which it was employed throughout 
my last memoir already referred to. If the surfaces of these ganoid scales, instead of 
being smooth, as is usually the case, had been fretted and sculptured with as infinite 
a diversity of minute points and ridges as are seen on those of Cycloid and Ctenoid 
fishes, I suspect that the former would have lost very much of the shining aspect to 
which they owe their name. It appears to me that there is no real difference between 
the superficial calcareous layer of a perch or a salmon, and the ganoin of a Lepidos- 
teus. This is, however, a point on which differences of opinion will doubtless exist ; 
I confess that it is equally diflScult to detect any difference between the same tissue 
as seen in the vertical section of a scale of a carp, and the laminee in some of the 
bones of a pike and other osseous fish, the bones of which contain no lacunae. In a 
subsequent portion of this memoir we shall find that the latter are merely the calci- 
fied lamellae of a periosteal membrane, exhibiting no visible traces of minute structure. 
The lamellae covering the scale of a carp are neither more nor less than this. 
Additional light is thrown upon this portion of our subject by the study of some 
other scales, to which I will now direct attention. One of the most curious of these 
belongs to a species of Balistes, one of the File-fish, in the collection of Sir Philip 
Egerton. 
In their external aspect these scales bear a close resemblance to those of many 
ganoid fish, exhibiting the rhomboidal form and peculiar tessellated arrangement 
seen in the members of that group, in which M. Agassiz has arranged it. On making 
a vertical section of one of these scales, we find that in its internal structure it ap- 
proximates closely to the type prevailing amongst the Cycloids and Ctenoids, retain- 
ing however one or two curious points of the resemblance to the Ostracionts. A 
representation of a vertical section, made parallel to the lateral line of the fish, is 
given in fig. 13 ; a is its anterior extremity, which is overlapped as far as h by the free 
margin of the antecedent scale ; c is its posterior border, the sloping inferior surface 
of which reposes in like manner upon the anterior portion of the scale behind it. As 
in the Cycloids, we have three very distinct horizontal tissues, lower, middle and 
upper. 
The lower one (fig. 13 </) consists of parallel laminae of membrane ; but in addition 
to its horizontal fibres, we here see that there are numerous thick fibres which pass 
obliquely upwards from layer to layer, binding them together; we shall afterwards 
find that these oblique fibres belong to a peculiar type prevailing amongst the Ostra- 
cionts, where they produce some very remarkable and beautiful structures. In the 
present example I do not perceive that their existence materially affects the conform- 
ation of the middle layer, fig. 13 e, which is a calcareous one, formed in precisely the 
same way as the corresponding layer amongst ordinary Cycloid and Ctenoid scales, 
viz. by the development of small round and lenticular granules within the fibrous 
tissues of the membranous laminae. In their aggregation these granules leave a 
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