446 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
ground away to render it more transparent, we find a beautiful arrangement of 
tubes or canals running immediately under and in the plane of the ganoin, form- 
ing, by their branches, a development of kosmine. The main channels (7 a) are 
slightly undulated, and send off to each lamina small lateral twigs (7 b), which, 
anastomosing with similar ones from adjoining tubes, form a series of loops, the 
arborescent terminal ramuli of which supply the parallel lines of the kosmine with 
nutriment. 
The subjacent laminse may be ground so thin as to exhibit no trace of either 
lacunae or any other kind of cavity or perforation whatever, appearing merely as a 
structureless calcareous layer separating two layers of lacunae and their parallel 
planes of canaliculi. There appears to be no way in which these horizontal laminae 
could receive their nutriment except through the lepidine tubes. The latter abound 
at each extremity of the section, where the laminae leave the horizontal to assume the 
upward direction : through the branches of these tubes the nutrient fluid might 
reach each layer of lacunae, and, by means of their canaliculi, be distributed laterally 
to every portion of the scale. The lacunae are somewhat larger than in Lepidotus 
semiserratus. I have found, that, however various may have been the dimensions of 
the scales under examination, there is but little difference in the size of their lacunae. 
The centre of each scale of P. Comptoni (7 c) merely exhibits a layer of these lacunae, 
and corresponding ones, following the plane of the upturned laminae, are seen edge- 
ways through the kosmine, 7 d. 
M. Agassiz describes the enamel {dmaiV) of Palceomscus as being nearly opake. 
It is, however, not more so than in any other genus. 
The scale of P. Beaumonti resembles that of P. comptus in its general features, but 
differs in points of detail. The large tubes or canals supplying the kosmine chiefly 
enter at the sides of the scale, Plate XLI. fig. 8 a. They do not terminate at the central 
rhomboid in the fine filamentous loops which characterise P. comptus, but some of the 
large tubes traverse this central portion, and communicate with corresponding ones 
entering from the other sides of the scale, 8 h. Along the upturned edge of each of 
the laminee, and parallel with it, are large transverse inosculating branches, 8 d, which, 
by connecting the main trunks together, form a network, from which are given off a 
vast number of minute branching filaments ; these are distributed to the thick layer 
of kosmine. On one side these anastomosing branches are very long, their extent 
being occasionally equal to the entire diameter of the scale, 8 e, owing to the entire 
absence, in this portion, of the large trunks which enter laterally. The very few 
which are visible, instead of being parallel with the ganoin, seem somewhat to ascend 
from below, 8 f. The same is the case with some of the main trunks at the angles of 
the scale, 8 c. 
Its anterior and posterior margins are freely supplied with lepidine tubes. 
No pencil can adequately depict the beauty of the filamentous branches of the 
anastomosing canals in the kosmine of this interesting scale. Though differing in 
