Pvn H,i I'UitlS

146 ARCIDM.
colouring. The first ground is evidently untenable,
because size is notoriously dependent on food, shelter,
and the proportionate quantity of carbonate of lime con-
tained in sea-water, according to its proximity to the
shore, or distance from it, and to the action of freshwater
and marine currents. The second ground, or the
degree of those angles
which affect the contour of
Nucula, is influenced by the growth of the shell in any
particular direction. All the Nuculce represent
an iso-
sceles triangle, the beak or point of one angle always re-
maining the same, while the sides or points of the other
two angles extend pari passu by the addition of new
layers in either direction. If one of these sides is more
produced than the other while the angle of each is
greater or less than that of the beak, a scalene triangle
is the result. This is the case with N. radiata ; and if
it were a permanent or invariable character, I should
regard it as having considerable weight in deciding the
question. But were the most acute mathematician to
measure the angles in certain specimens of the two so-
called species, he would assuredly fail to detect any
sensible difference. As to convexity, it is true that specimens
of N. radiata are usually more compressed than
those of N. nucleus. Other specimens, however, of both
species are equally convex. The coloured rays are
clearly a varietal, and not a specific character. They
are even more conspicuous in another species (N. nitida),
which has commonly a plain and sober hue. Instances
of a similar diversity in this respect occur in Astarte
triangularis and many other marine shells, as well as in
species of the freshwater genus Unio. The nature and
cause of colour in shells has not yet received that atten-
tion from philosophical chemists which the interest of
this curious subject demands. Probably
the most in-