448 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
tion of the periosteal membrane corresponding to that which forms the ganoin in the 
other scales. 
Acipemer . — Sections of the scale of the common Sturgeon present a structure some- 
what similar to that existing in the Aspidorhynchus, only the numerous ascending 
parallel tubes are wholly wanting, and the process of involution of the upper portions 
of the lamellae has been carried to a much greater extent, leading to the production 
of true Haversian canals. 
When one of the large lozenge-shaped scales is examined, its upper surface is found 
to be rough and thorny. This is owing to the existence of what at first sight would 
appear to be a deposition of something resembling ganoin upon the true bony tissue : 
this hard substance, which covers the free portion of each scale, has a tendency to be 
arranged in the form of radiating ridges, extending from the centre to near the cir- 
cumference of the scale, excepting on the anterior portion, which, having supported the 
opposite margin of the preceding scale, is quite smooth, as is also the inferior surface. 
In the central portion of the upper surface the radii are less regular than towards 
the posterior edge of the scale, being more cribriform in their aspect, from the exist- 
ence of numerous irregular pits and deep depressions which exist in it. A smooth 
elevated ridge crosses the centre parallel with the lateral line ; this covers over a very 
large canal, the superficial opening of which is in the smooth anterior portion of the 
scale, which has been covered over by the upper fold of the integument : its opposite 
extremity is at the under surface of the scale, near its posterior border. The canal has 
obviously transmitted blood-vessels, and probably nervous twigs also, keeping up a 
free communication between the two portions of the integument. Several smaller 
but analogous canals communicate between the upper and lower surfaces, each of 
them verging towards the centre as it ascends. 
The lower surface is very smooth and translucent, exhibiting a series of concentric 
lines, like those in the interior of a bivalve shell, and which at first glance might 
lead to the idea that the enlargement of the scale had been accomplished, as in the 
shell, principally by the addition of new matter to its edges. Such, however, is not the 
case. These concentric lines are in reality only the points at which the successive 
laminae constituting the inferior portion of the scale turn upwards and inwards at a 
very acute angle, as seen in the section, fig. 11a: these lines of course represent what 
were from time to time the external boundaries of the scale, which were enclosed by 
successive new growths. 
As we have seen to be the case in Aspidorhynchus, soon after leaving the horizontal 
condition, the upturned laminae present a strong tendency to undulate, but even to 
a much greater degree. Near the margins of the scale, these undulations only pro- 
duce alternating grooves and ridges on the surface. This appears to be the portion 
figured by M. Agassiz* ; whose representation, however, gives a very imperfect idea of 
its true structure. But as the section approaches the centre of the scale, we find that 
* Poissons Fossiles, vol. ii. Tab. H. fig. 22. 
