SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISH. 445 
nicating between the two surfaces of the scale — the parallel ascending tubes, which, as 
in Lepidotus semiserratus, teriniuate in a considerable development of kosmine, the 
elevated ridges of which project into the ganoin. The anastomosing loops of the 
kosmine are in every way larger and stronger than in any species hitherto described, 
the numerous branches and twigs which they give off from their elongated arches ex- 
hibiting the appearance of cJievaux-de-frise. 
Dapidius. — In Dapidius orhis, the lamellse of the scale are exceedingly distinct 
and beautifully parallel, but the division of them into laminae is less obvious. We 
have the large canals communicating between the two surfaces, causing the puncta 
on the exterior of the scale, and the parallel tubes as in the Lepidosteus. Tlie lepi- 
dine tubes are very extensively developed, especially at the anterior margin of the 
scale. The ganoin is remarkably thin, and in some examples scarcely visible. 
In Dapidius grmmlosus, the substance of the scale has the same structure as that 
of D. orhis, but its surface is studded over with scattered raised points, one of which, 
as seen through the superficial ganoin, is represented in fig. 5. Each one forms an 
elevated point, the posterior portion of which (a) exhibits a defined convex edge. It 
is covered with a layer of ganoin, which is but slightly developed over the intervening 
layer of the scale. This is seen in fig. 6 a, which represents a vertical section of one 
of these tubercles, made in the direction of the dotted line, fig. b c c. Under each of 
these exists a small cavity, figs. 5 b and Qb, which opens externally, by means of 
three or four small canals, fig. 5 d, the orifices of which are placed behind the tubercle. 
Above this cavity we have a development of kosmine, the tubes of which chiefly arise 
from the anterior side of the cavity, 6 e, 6 c. Thus we find that each of these tubercles 
consists of a local development of kosmine and ganoin upon the upper laminae of the 
scale, fig. 6 d, these latter being constructed on the ordinary type seen in so many of 
the lepidoid scales. To this limitation in the distribution of the kosmine and ganoin 
we shall have to refer again, as it constitutes one of the earliest forms, in which the 
tendency existing in many fish to the development of dermal teeth-like structures, 
manifests itself. 
Palceoniscus . — This genus, of which I have investigated two species, the P. comptus 
from the magnesian limestone of Durham, and the P. Beaumonti from Autun, is 
allied in many respects to the group already noticed. In the arrangement of the 
laminse and lamellae, the scales of the above species exhibit the general aspect of the 
Lepidoids already described, but they materially differ in the distribution of their 
system of internal canals. 
On making a vertical section of the scale of P. comptus, parallel with the mesial 
line, we see none of the long parallel tubes traversing the laminae at right angles, 
which form so conspicuous a feature in the scales hitherto described. In other re- 
spects, the laminae, fig. 7 e, both in their horizontal and upturned portions, as well as 
the lacunae and their canaliculi, agree with those of Lepidotus and its allies. 
On looking through the ganoin, when the lower portion of the scale has been 
3 M 
MDCCCXLIX. 
