456 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
One question arises to which I am unable to give a decisive answer. May there 
not have been in this scale of Megalichthys, a central nucleus of cartilage in the 
midst of the Haversian canals, in which the first deposition of calcareous matter may 
have taken place, and upon which the horizontal lamellae have been subsequently 
added by the ordinary process of intramembranous ossification ? This is just possible, 
though we have no evidence of its truth ; whilst the scale of a Holoptychius, shortly 
to be described, and presenting a closely allied structure, is opposed to the supposi- 
tion, and supports the idea, that the scale of Megalichthys, complicated as it is, has 
been wholly formed by the successive organization and inflexion of layers of mem- 
brane in which the granules of calcareous matter have been subsequently diffused. 
Diplopterus. — M. Agassiz has already examined some species from the old red 
sandstone. He remarks, “ Les ecailles presentent une fine granulation provenant 
d’une quantite de petits trous qui s’ouvrent de passage pour les nombreux petits 
vaisseaux sanguins qui traversaient I’ecaille pour se rendre dans fepiderme. Exa- 
minees au microscope, les Readies presentent une epaisse couche d’email, au dessous 
de laquelle se trouve un tissu osseux montrant des reseaux fort elegants, qui ne dif- 
ferent de ceux de Polypt^re que par leur developpement considerable. Les trous et 
les canaux medullaires I’emportent de beaucoup sur les piliers intermediaires*.” 
Fig. 20 represents a horizontal section of a very thin scale belonging to an unde- 
scribed species from the coal-field near Leeds. The original specimen was about 
half an inch in length, and, as in Megalichthys, was covered with shining ganoin, 
which was perforated by innumerable minute apertures, the orifices of canals. The 
section was made at a slightly inclined angle to the plane of the scale, so that whilst 
the extremity a cuts obliquely through the superficial ganoin and its subjacent kos- 
mine, the opposite end b, especially to the right-hand of the figure, dips more deeply 
into the bony tissue of the scale. Though I have not been able to procure a second 
example of this scale, in order to make a vertical section, there is no difficulty in 
reading off its beautiful structure, and comparing it with the vertical sections of 
Megalichthys and Holoptychius. It corresponds exactly with what has already been 
described as the immature condition of the former, and closely resembles that pre- 
sented by one species of the latter. 
c is the superficial layer of the kosmine supporting an exceedingly thin film of 
ganoin ; the dark portion d is the horizontal cavity, traversed by the hollow pillars 
of kosmine, e, which surround the trumpet-shaped descending cavities: these com- 
mence by small apertures in the superficial layer,/*; at e they gradually enlarge, be- 
coming angular at g, and at h giving off the minute connecting tubes i, which cor- 
respond with those of the Megalichthys, fig. 15 c. Below this the descending cavities 
become lost in the ramifications of the Haversian canals I, as at h. The small tubes 
i divide the kosmine into areolar spaces, and into the centre of each there arises an 
offshoot from the Haversian canals, m, opening superiorly into the horizontal cavity d, 
* Poissons de Vieux Gr^s Rouge, p. 54. 
