458 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
centric ridges are traversed at the two extremities, but especially at the posterior 
one, b, by minute radiating lines ; at the two sides of the scale they are not visible. 
On the inferior surface the concentric lines extend over two-thirds of the scale, fig. 22, 
but they are less regularly definite and uniform in their thickness than on the oppo- 
site side. We find none of the radiating lines which M. Agassiz noticed in the corre- 
sponding portion of the scales from the old red sandstone, but its acuminated extre- 
mity, 22 a, corresponding to 21 a, exhibits a number of large puncta, 22 c, which are 
the orifices of ascending canals. On making a vertical section of one of these scales, 
I found few traces of either kosmine or ganoin ; it consisted of numerous lamellae, 
the lower ones resembling those seen in the corresponding portion of Megalichthys. 
These are shown in fig. 23 a, which represents a vertical section of a large scale, 
taken in the direction of the dotted line c in fig. 22. These lamellae are furnished 
with lacunae and canaliculi like those of Megalichthys and Diplopterus, fig. 19. The 
upper portion of the scale consists of the upturned lamellae, which by their inflexions 
form the ridges which ornament its external surface, 23 b. In the section represented 
in the Plate, these ridges are less striking and prominent than ordinary ; generally, 
instead of the section presenting a gently undulating outline, these ridges are irre- 
gular and even overhang the furrows which separate them, but still consist of the 
inflected extremities of the lamellae, as do also the fine radiating lines of fig. 21. 
Under these ridges, at the anterior part of the scale, we find a series of concentric 
canals, connected together by short anastomosing branches. They do not follow any 
very uniform direction in their distribution, varying considerably in the details of 
their arrangement, always however showing a tendency to be regulated by the direc- 
tion of the lamellae themselves, they having evidently been formed on the principle 
seen in the scale of the Sturgeon ; thus in the section fig. 23, we find that many of 
the branches of these canals exhibit a curvilinear arrangement, 23 c, their direction 
corresponding with that of the lamellae. From these Haversian canals are given off 
numerous vertical branches, especially at the acuminated extremity of the scale. 
Those which ascend, open in the grooves separating the concentric ridges, fig. 23 d, 
whilst the orifices of the descending ones produce the puncta seen at fig. 22 c. These 
vertical canals have not been formed by inflexions of the lamellae, but by the leaving 
out of the apposite portions of each succeeding lamella as it was organized ; con- 
sequently they merely pass through the latter nearly at right angles to their plane. 
The whole texture of the scale is crowded with various modifications of lacunae, 
from the fusiform ones already described to those of the ordinary ichthyal type ; and 
though many of their canaliculi traverse the lamellae, they are chiefly developed 
parallel with these layers, and follow their direction. 
In the species examined by M. Agassiz, that philosopher found a structure very 
similar to the one just described ; he applies the term enameled {dmailUe) to the 
layers constituting the external ornaments of the scale, at the same time however 
observing, that they are “ qu’une substance osseuse plus epaisse, dans laquelles les 
