DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
657 
with the examples just cited, these permeating and anastomosing canals. In their 
respective classifications, Agassiz, Muller and Owen have alike recognized the 
“ganoid” character of the scale of Balistes; hence this “ganoid” structure, homo- 
logous as it is with the superficial tissue of the scale of the Carp, becomes a strong 
argument in favour of my idea that the uppermost layer of the Cycloid and Ctenoid 
scales is identical in its character with the ganoin existing on those of Lepidostem, 
Lepidotus, and their allies. 
We obtain an additional illustration of the way in which these structures are mo- 
dified and linked together from the scale of the Flying Gurnard {Dactylopterus voUtans, 
Lacep.). Anteriorly these scales present a thick and expanded base, but posteriorly 
they taper away to a thin and narrow point. In the interior of the scale is a number 
of large irregular cavities with connecting channels of communication, and which 
give off numerous irregular anastomosing canals, running in every direction. On 
making a vertical section through the centre of the scale, parallel with the lateral 
line, we obtain the result represented in fig. 14. 
With the exception of a slight modification of part of its inferior surface, this scale 
is obviously composed of one uniform tissue, which approximates more closely to 
the common form of bone as it exists amongst the osseous fishes than in any other 
example with which I have met. In its essential features it also bears a close re- 
semblance to the uppermost layer of the scale of BaUstes, fig. 13 f. It consists of 
% 
numerous translucent laminae variously inflected; the external ones lying parallel 
to the contiguous surfaces, whilst the internal ones follow the outline of the large 
central cavities which they invest. They are perforated by the canals, Plate XXIX., 
fig. 14, a,h,c, which run in every direction and communicate with the various sur- 
faces; they are prolonged alike into the thin anterior basilar expansion, 14 d, which 
is implanted in the soft integument and into the posterior acuminate extremity of the 
scale, 14 e. Inferiorly, the horizontal lamellse (14 f) are perforated by a number of 
branching tubes, which coalesce as they ascend, and finally open into the large central 
cavity, converting this portion of the scale into a tissue resembling kosmine. 
If the uppermost layer of the scale of BaUstes be really ganoin, as thought by 
M. Agassiz and Professor Owen, and of which I have no doubt, then nearly the vvhole 
of the scale of Dacty lopierus consists of the same tissue. I am unable to see any 
essential difference between the two examples. On the other hand, if we compare 
the structure of this scale with that seen in all the bones of the endoskeleton of the 
Pike, and especially with the central portions of the epitympanic (fig. 39) or the arti- 
cular extremity of the opercular (fig. 40), we shall find that no essential difference 
exists between them, beyond a slight variation in their respective densities and degrees 
of hardness. In both we have the same inflected membraniform lamellse, surround- 
ing similar central cavities, and perforated by analogous Haversian canals ; whilst a 
very considerable degree of resemblance exists as to their respective modes of growth, 
both being produced, as we shall afterwards find, by the analogous calcification of an 
