SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 459 
couches sont efFacees, et les corpuscules plus grandes.” M. Agassiz also notices 
radiating lines crossing the concentric ones, but he describes them as forrnees par 
de petits canellures tres fines et a peine en relief, dans lesquels se fixaient probable- 
ment les fibres de la peau.” In H. sauroides, as we have seen, the inferior surface ex- 
hibits none of these lines ; and in the upper layers the lamellae are not effaced, though 
the structure is dense. I have not unfrequently observed, in some of the projecting 
ridges, a slight disposition towards the development of kosmine tubes, as if nature 
was making her earliest efforts at converting the true osseous lamellae into 
kosmine. 
The preceding description, which applies to the majority of the scales of Holopty- 
chius which I have examined, reveals to us many points of remarkable identity be- 
tween them and those of Megalichthys and Dlplopteriis, indicating a much closer 
afiinity between these three genera than has hitherto been recognised. The exami- 
nation of one oblong scale belonging to an undoubted species of Holoptychius from 
the upper coal shales of Lancashire, establishes this affinity still more strongly. Its 
inferior surface exhibited the same appearance as H. sauroides ; smooth concentric 
lines existing at its rounded extremity, whilst the acuminated one was studded with 
large puncta. But on making a vertical section, a striking difference presented itself 
in the superior surface, which, being adherent to its matrix, could not previously be 
seen. The rounded extremity exhibited the structure seen in the corresponding 
part of H. sauroides. The inferior layers of the opposite extremity also correspond, 
fig. 24 a. The puncta open into ascending canals, which perforate the laminm, 24 b, 
and communicate superiorly with a system of canals or cavities, 24 d, analogous to 
those of the Diplopterus, fig. 20 d, to the appearance that would be presented by a 
vertical section of which this form of Holoptychius forms an excellent illustration. 
Above and around this superficial cavity, 24 d, is a development of kosmine, which is 
penetrated from above by trumpet-shaped cavities, 24 e, and which give off small 
connecting tubes, 24 f, transverse sections of which, coming from the more distant 
cavities not cut across by the section, are seen at g. These trumpet-shaped cavities 
are not quite so gracefully formed as in Megalichthys, but in other respects they are 
very similar. After giving off these tubes, the cavities spread out continuously in 
every direction over the osseous tissue, and send up into each areola formed by the 
network of tubes, an expansion, 24 h, analogous to the cul-de-sacs of Megalichthys, 
but which, instead of being isolated as in that genus, in the mature state of this Ho- 
loptychius all open into one another, as seen at the extremity of the scale, d-, the only 
connecting portions between what may be regarded as the roof and the floor of this 
space being the hollow pillars surrounding the cavities, e. As in Diplopterus, the 
whole of the kosmine receives its minute tubuli from this large superficial space, from 
which they radiate in every direction. 
This section also explains what has been already said respecting the growth of the 
kosmine in Megalichthys. Its permanent structure in Holoptychius presents the con- 
dition which has obviously existed in the young state of the scale in that genus. On 
