43 
with Observations on their Structure and Colour . 
I have further observed, that the external or first-formed coats 
of several shells, for example the Pinna marina, have the fi- 
bres much coarser than the inner or last-formed coats ; and I 
have also observed, in a species of molluscous animal, that the 
internal shell, which is -formed as a shield for the protection of 
the heart and viscera of that animal, passes from the soft and 
membranous state to that of a shell, in a manner similar to the 
ossifications visible in those who have died 01 hydiocephalus, 
where many round points or islands of bone extend their edges 
in every direction, to unite and foim a solid crust.. 
The difference between the fresh-water mussels of different 
streams in the same neighbourhood is very remarkable. I know 
that there are different species of mussel inhabiting the same 
river, for example, the river Severn, at Worcester, in which, I 
believe, there are at least three species. 
In one of the species, the IJnio pictorum, I have observed, 
j w hile the animal was living, very deep erosions on the outside 
of its shell, and nearly in the most convex part of it. Might a 
consciousness of these erosions, (that is, of this attack upon its 
| habitation), have given rise to the extraordinary thickness al- 
ways observable in such diseased shells, or was this appearance 
merely the result of decay from old age ? The shell in such 
cases easily exfoliated, and these eroded cavities, which were 
nearly circular, were often lined with a bright green colour, of 
which I had no opportunity of examining the origin. Another 
accident to which this species is liable (or at least which I have 
seen in no other), is the apparent hasty formation of the inner 
coat of the shell.. At the time when I took the animal, almost 
the whole of the inner surface of the shell was of a livid silvery 
colour, and this last coat was not in immediate contact with the 
former coats, but rather resembled a large blister, with an un- 
dulated surface. On breaking this inner coat, I found in the 
cavity a considerable quantity of sand of the river ; and, though 
the two shells of the same animal had not the disease in the 
same degree, yet, if I found this accident very conspicuous in a 
shell, I frequently found a tendency to it in the opposite shell, 
shewing itself in greenish and dusky coloured spots; whence, 
perhaps, the whole is a disease of constitution, or a weakness of 
ossifying power, dependent upon old age« 
