440 MR. W. C. WILLIAMSON ON THE MICROSCOPIC STRUCTURE OF THE 
when their distribution is confined, as we shall occasionally find it to be, to particular 
portions of the scale ; they always penetrate every one of the lamellse, and thus com- 
municate with each parallel layer of lacunae, through the canaliculi of which the 
necessary lateral communication can be carried on^. 
There is in addition, a third set of still larger, though less numerous canals, which 
pass completely through the scale, fig. 1 g. These are supposed by M. Agassiz to 
convey the blood through the scale in order to supply the epidermal layer by which 
it is covered. Existing, as we shall subsequently see, in nearly all the fossil species, 
they obviously play an important part in the economy of the scale, by keeping up a 
free communication between its upper and lower surfaces. They are not confined to, 
or even chiefly found in, the anterior portion of the scale, which, when in situ, is em- 
braced by the thick duplicature of the skin, but in that part of it which is free, and 
which has beyond doubt been covered over with a thin secreting membrane like a 
periosteum, and which has received some of its supply of blood through these open 
canals, by means of anastomosing blood-vessels, ascending from the integument 
below. About the posterior two-thirds of each scale is covered over with a layer of 
ganoin. In some parts this is so thin as to be scarcely visible, figs. \h and 2f, 
whilst in others it is developed into irregular tubercles, fig. 2g. From the latter 
representation, we see the way in which these tubercles of ganoin are formed. They 
are, in fact, prolongations of the upturned edges of the bony lamellse, running 
towards the centre of the scale, but only covering small portions of the surface, 
instead of uniformly extending over the whole, as we shall afterwards find to be the 
case in Lepidotus and other fossil forms. We see from this, that each lamella of the 
ganoin was formed contemporaneously with that of the bone from which it springs; 
each lamina, shown in the drawing, consists of a number of more minute lamellse, 
as has been already observed in reference to the osseous portions. With the excep- 
tion of the tubes, of whicli the orifices have been already alluded to as perforating' its 
laminse, I have not been able to detect any other microscopic structure than these 
lamellse in the ganoin. 
1 subjeeted some of these scales to the decalcifying action of dilute hydrochloric 
acid, and obtained a dense flexible tissue, preserving all the original contour of the 
scale. In this were still exhibited the three sets of canals or tubes, and the lacunae 
with their canaliculi. The traces of the lepidine tubes were to be seen so crowded 
together as apparently to compose almost the entire tissue of the scale. Sections 
taken at right angles to these tubes exhibited very similar appearances to what are 
seen in a corresponding section of the decalcified tooth of a Cachalot, or any other 
* These tubes appear to correspond with those to which Prof. Owen has applied the term “plasmatic.” 
But as he includes under this title all those commonly known as canaliculi, radiating from the lacunae, which 
are obviously distinct in their nature, I think it will serve the purpose of rendering our descriptions more clear, 
if we employ a new term to distinguish those which I have designated “lepidine” tubes. See the Lectures 
on the Comparative Anatomy and Physiology of the Vertebrate Animals, by Prof. Owen, part 1, p. 28. This 
eminent anatomist appears to have noticed them in the scale of the Sturgeon. 
