Pvn H,i I'UitlS

MONTACUTA. 215
become coated with a thick but irregular layer of hard
ochreous dirt, like iron-rust. I suspect that this in-
crustation is caused by a continual deposit and accumu-
lation of faecal matter from the animal, which is not
carried off in consequence of its sedentary habits and of
the water in which it lives being free from currents.
The same remark applies to species of Pisidium and
other freshwater shells, as well as to many of the ma-
rine bivalves. Forbes and Hanley observe that M.fer-
ruginosa is " a scarce shell." This of course is only
in comparison with other species; but the common
notion of rarity is not quite correct. I consider that
" local" would be a more appropriate word than "scarce"
with regard to the occurrence of marine testacea. The
difficulty of procuring some particular kinds may enhance
their value to collectors ; but probably all kinds
are prolific, and differ in that respect from many of the
larger land mammals. There certainly cannot exist a
"
unique" specimen of any shell in nature. In the
present case single valves may
be found in tolerable
plenty on several of our sandy beaches, and perfect specimens
may be got by dredging in the sheltered lochs of
the west of Scotland and in rather deep water off our
southern coasts. The beaks are often fissured or notched
from their being squeezed close together by the strong
and elastic cartilage, the shell being too fragile to resist
such pressure.
The proportions of length and breadth vary greatly
in specimens, which may account for the diversity of
names which this species has from time to time re-
ceived. As I have already incidentally noticed, it ap-
pears to be the Amphidesma purpurascens of Lamarck ;
Brown called it Tellimya elliptica and T. glabra,
S. Wood T. ovata, Leach Amphidesma Goodalliana,