SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 457 
which separates the upper layer of the kosmine from the tissues below. As vve 
have already seen to be the case with Megalichthys, neither the descending cavities 
€,g, nor the small tubes i into which they subdivide, give off any minute tubuli, but 
their walls are wholly supplied either from the superficial space d, or from the cavity 
in the centre of each areola, m. The latter especially give off branching kosmine 
tubuli of considerable size and great beauty. In the former the tubuli are very small, 
with the exception of those which ascend to the superficial layer c, which are thicker 
and more branched. In the hollow pillars, e, these tubes are uniformly parallel, ra- 
diating inwards. As in Megalichthys, all the tissues in the plane of and above the 
small inosculating tubes i, consist of kosmine, excepting the thin superficial layer of 
ganoin. 
The tissues surrounding the Haversian canals, which are rather large, are osseous, 
presenting the same appearances as those of Megalichthys. The inferior laminae of 
the scale also are horizontal and parallel, presenting the characteristic fusiform or 
linear lacunae represented by fig. 19. 
A vertical section of another scale already alluded to, also from Leeds, slightly 
differs from the last. The upper layer of the kosmine is thicker, and from the vertical 
cavities entering it but a small distance before giving off the inosculating tubes, 20 i, 
it would be impossible by any horizontal section to exhibit the elegant rings seen 
in 20 e. The Haversian canals are more like the large cancelli in the diploe of bone, 
and the branches which they send up into the kosmine are equally large ; illus- 
trating the description given of the development of the scale of Megalichthys. 
Holoptychius . — The structure of some scales from the old red sandstone, belonging 
to this genus, has been already described by M. Agassiz*. His results, however, 
differ in many material points from those obtained by my own observations upon 
scales belonging to the same genus from the upper coal-measures of Lancashire, where 
at least two, if not more, species exist, which have hitherto been confounded under the 
name of H. sauroides. 
These scales vary from being nearly orbicular to being so elongated, that their larger 
diameter becomes three or four times greater than the opposite one. In all cases one 
extremity is more pointed than the other, the latter being not unfrequently cordate. 
Their inferior surface is usually the only one seen, the upper one being adherent to 
the matrix. Fig. 21 represents the usual aspect of the latter, and fig. 22 of the former, 
amongst the larger scales of H. sauroides. Both surfaces exhibit the concentric lines 
noticed by M. Agassiz as ‘‘ repetant les contours de I’ecaille.” These are the most 
beautifully regular and definite on the upper surface, especially at its anterior extre- 
mity, 21 a, but towards the pointed end, h, they give place to others of a larger size, 
but which are less numerous as well as less regular in their distribution. When 
the scale was in situ, the latter occupied the exposed portion, the remainder being 
covered over by the pointed extremities of the two scales in front of it ; these con- 
* Poissons de Vieux Gr^s Rouge, p. 70, tab. 24, fig. 10. 
