THE ANATOMY OF THE EIVER-MUSSEL. 
243 
regular in shape, and are imbedded in the soft parts of the 
animal.* 
The French naturalist, Lacaze-Duthiers, having found in 
the ovary of a Mactra (one of the Lamellibranchs) whose renal 
organ contained concretions of uric acid, an ovum which in- 
cluded between its shell and yolk a veritable calculus, suggests 
that pearl-producing molluscs are in a condition like unto that 
of a gouty man ; seeing that, as in the latter case, concretions 
of uric acid or its salts are apt to be formed in the bladder or 
around the joints, so, in the former instance, the excess of 
material over that which would normally suffice for the wants 
of the shell comes to be precipitated in the form of a pearl. 
“ En un mot, le mollusque produisant des perles, n’est-il pas 
un etre atteint d’une diathese calculeuse?” 
The true basis of the shell, a very small portion of which 
can be seen running round outside the edge of the nacreous 
layer, and which has a somewhat roughened appearance when 
viewed through a hand-lens, consists of a layer — the “pris- 
matic” — made up of carbonate of lime deposited in elongated 
cellular cavities disposed vertically to the plane of the shell, 
and which are the result of a successive piling up of perforated 
laminae, the polygonal holes in which correspond, or “ coin- 
cide,” as Euclid says, secreted periodically by the then thick- 
ened margin of the mantle. 
A structureless epidermis — the “ periostracum” f — consisting 
of organic matter only, covers the shell-tissue and protects it 
from the solvent action of the carbonic acid with which fresh 
waters are more or less charged. It is connected with the 
edge of the mantle, being secreted, like the prismatic layer, by 
it, and is, in certain Lamellibranchs possessing siphons, e.g. 
the “ gaper” ( My a), continued over these organs. In Anodonta 
it is ornamented with concentric ridges corresponding to the 
lines of growth ; but in another kind of fresh-water mussel — 
the Dreissena — strictly a native of Russia, but now natural- 
ised, after emigration on foreign timber, in some of our rivers 
and canals, these lines radiate from the umbo to the circum- 
ference of either valve. 
Professor Rolleston, from observing that the shells of Ano- 
donta from the rivers around Oxford are thinner than those 
from mountain streams, though the latter waters are poorer in 
salts of lime than the former, infers that the amount of inor- 
* In the educational series of Invertebrata contained in the Museum of 
the Royal College of Surgeons, there is a preparation (No. 60) of a Unto 
which has a well-shaped pearl, of about the size of a pea, imbedded in the 
right side of the foot. 
t From TrEjoi = around, and oarpaKov = a shell. 
