SCALES AND DERMAL TEETH OF SOME GANOID AND PLACOID FISK. 449 
by the deposition of new layers, these grooves are covered over and converted into 
true Haversian canals. As we should expect to be the result of this mode of growth, 
none of the laminae forming these canals exhibit complete circles; the involutions 
of the lower layers are met by corresponding ones in the upper laminae ; and by the 
juxtaposition of their respective salient points, the canal is rendered complete. Thus 
in fig. ] 1, the depressions h are quite ready to be covered over in the same way as the 
completed canals c, and doubtless, in time, the addition of new laminae would have 
so closed them in. The whole of the scale is abundantly supplied with lacunae and 
their stellate canaliculi, though in the inferior horizontal portions of the lamellae 
these are not so beautifully distinct as in those surrounding the Haversian canals. 
The latter portions are also freely supplied with minute branching lepidine tubes, 
which descend from above, and, with the exception of the large canals already 
described, appear to be the only tubes the scale contains*. This beautiful illustra- 
tion of the way in which Haversian canals may be formed is one of great value to the 
physiologist ; because, from the size and distinctness of the laminae, and the ease 
with which their direction can be traced, they leave no possible room for doubt on 
the subject. We thus derive, from this comparatively remote source, strong corro- 
borative evidence of the accuracy of Professor Sharpey’s views respecting the origin 
and mode of formation of the analogous structures in human bones. 
Platysomus . — Constructed on the same principle as those already noticed, but 
exhibiting a very curious modification of it, is the scale of Platysomus parvulus, from 
the upper coal-measures of Leeds and Manchester. 
The exposed part of the upper surface of this scale is covered with deep grooves 
and intervening ridges, running nearly parallel with its long axis, which, as in Gyro- 
dus, really represents its breadth, and is at right angles to the direction of the lateral 
line of the fish. 
On making a section of the scale in the opposite direction to that taken by these 
ridges, we find it to consist of two portions, an upper and a lower one. The latter, 
figs. 12 a and 13 a, though apparently of a dense homogeneous structure, exhibits, on 
a careful examination, clear evidence that it consists of a series of minute lamellae, 
though these do not appear to be aggregated into any more conspicuous laminae. In 
the upper portion, we find that each elevated ridge consists of a series of concentric 
arches, fig. 12 b, having intervening crescentic spaces, 12 c, and exhibiting traces of 
the existence of canals, connecting one set of arches with another. On making a 
section in the opposite direction, along the line of one of these elevated ridges, fig. 13, 
this strueture is more fully explained ; and each individual ridge shown to be formed 
by a series of arehing plates, 13 b, which arise from the compact portion of the scale, 
13 a, and after successively overlapping each other, losing themselves at the upper 
surface of the scale. In fig. 14, which represents a horizontal seetion, taken in the 
* These were noticed by Prof. Owen, and recognized as belonging to his “ plasmatic series.” See Lec- 
tures on the Vertebrate Animals, part 1, p. 31. 
