Pvn H,i I'UitlS

NUCULA. 147
structive and elaborate examination of this problem will
be found in the essay of Professor Lacaze-Duthiers,
entitled "Natural History of the Purple of the Ancients/'
to which I have referred in the Introduction to the first
volume (p. lxvii), and which I shall again have occasion
to notice. There seems to be no doubt that all colours
are of mineral origin, and that they are secreted, by the
Mollusca and other animals, from inorganic matter hj
special glands. But there is no necessary connexion
between the colour of the mollusk and that of its shell.
The plain variety of our native Cowry (Cypraa Europcea)
is a familiar illustration of this fact. While the shell
is of a uniform porcelain-white colour, its inhabitant
and fabricator exhibits most varied and brilliant tints of
vermilion, yellow, brown, green,
and red. The coloured
markings of shells cannot be relied upon as a specific
test, especially when the same hue predominates; and
N. radiata is in this respect undistinguishable from
N. nucleus. Taking into consideration the question of
with reference to the remarks which I made in
locality,
the Introduction to Vol. I.
(pp. xix and xx), I believe it
will be found that the two forms do not live together.
I have never taken them in the same spot ;
and Forbes
and M.'Andrew have observed that N. radiata occurs
at Milford Haven " always in separate parts of the bay
from nucleus" This circumstance would of itself in-
cline me to doubt the propriety of separating these forms
unless as varieties. Sufficient weight does not seem to
have been given to the remarkable occurrence in the
same locality of different species of marine animals
which are bisexual but require mutual impregnation *.
* Oppian speaks of this mode of generation with abhorrence :—
Eire irpbs dXXrjXcov, repas dypiov, eKcpvovrai,
Notri/u 7r69iov, Kai vbatyi fa\ui)v, na\ vbatyi tokoio.
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