660 
PROFESSOR WILLIAMSON ON THE STRUCTURE AND 
the types of several groups of scale structures are blended together in the latter fish. 
Whilst upon a truly cycloid scale are fixed numerous placoid teeth, in its perfectly 
ossified and kosmine-covered opercula, it approaches to Dapedius granulosus and other 
allied forms belonging to the ganoid order of M. Agassiz. Such is the extraordinary 
way in which the few primary elements entering into the composition of the ichthyal 
skeleton, are modified and combined in order to produce the greatest possible di- 
versity in the resulting structures ! How impossible must it be for any truly phy- 
siological system of classification to be framed, if it is based only on the relative 
degree in which any one of the elements composing the dermal skeleton are developed ! 
I have now to describe some still more singular and beautiful textures existing in 
the scales or dermal plates of fish belonging to the genus Ostracion. It is well known 
that their exoskeleton is composed of numerous small angular pieces, which do not 
overlap each other, as is the case with most other dermal scales, but are fitted side 
by side like the tiles of a tessellated pavement, on the plates constituting the shell of 
an Echinus. I am not aware that their structure has been subjected to any investi- 
gation beyond that of M. Agassiz, briefly noticed in the Poissons Fossiles, where, he 
observes, “ la couche inferieure est une substance comic, deposee par lames, et affec- 
tant des formes tr^s-diverses dans la meme 4 caille ; elle est recouverte d’une couche 
epaisse de dentine tres-bien caracterisee par ses tubes calciferes ramifies, qui ressem- 
blent en tout aux tubes calciferes des dents.” The idea which this distinguished 
savant had adopted as to the horny nature of the fibrous portions of Cycloid scales, 
has again biassed his judgement and led him into a similar error in interpreting the 
appearances presented by the dermal plates of Ostracion. He gives a sketch* of the 
vertical section of a scale of Ostracion triqueter, the leading outlines of which are 
very correct, but the more minute structural details are not delineated. 
Fig. 17 represents the upper surface of one of these scales (double the natural size), 
for which I am indebted to J. E. Gray, Esq. of the British Museum. As in Batistes, 
this surface is usually covered with large tubercles. The inferior surface on the 
other hand is smooth, and exhibits numerous concentric lines, like those represented 
by fig. 18 ; only towards the centre of the scale the angular concentric lines become 
less distinct than in this section, owing to the intervention of successive laminae of 
fibrous membrane. Numerous minute apertures, which exist in these laminae, are the 
orifices of canals which pass obliquely upwards towards the centre of the scale. 
Fig. 19 represents a vertical section of one of these structures made in the direc- 
tion of the dotted line, fig. 18 au! . On looking at the latter drawing, it will be per- 
ceived that, whilst in its central portion the section runs parallel with one of the 
concentric lines a little to the right of the centre of the scale, at its two extremities 
it more or less directly traverses those of the two contiguous series. 
From the above section, we learn that this curious scale is partly calcareous and 
partly membranous; the two substances being arranged in alternating horizontal 
* Poissons Fossiles, vol. i. tab. H. 
