Pvn H,i I'UitlS

148 ARCID.E.
The difficulty arising from the sterility of hybrids among
unisexual animals being thus removed, how does it
happen that so many distinct but allied species assemble
and live together without interbreeding or mingling
their respective races ? There is no fusion, there are
no intermediate forms or gradations from one to an-
other; each pursues its own course and mode of life,
and appears to have no communion w T
ith its neighbour.
Surely
this emboldens naturalists to maintain the in-
tegrity of species as at present existing,
however much
they may have changed in the course of bygone ages.
We take Nature as it is and apparently has been— not as
it might have been ; and all speculations as to the origin
of species, although ingenious and interesting, are useless,
for want of sufficient data to guide us in the inquiry.
The variety tumidula resembles N. nitida in shape ;
but the surface is not glossy as in that species, and the
is the same as in N. nucleus.
sculpture
The present species seems not to be liable to distor-
tion ; but a valve now before me from Guernsey has a
fold on the posterior slope, contracting the shell, and
somewhat resembling the sinuosity of Axinus fleocuosus.
Specimens
from the Hebrides and the south-west of Ire-
land are larger than any I have seen from other parts,
whether British or foreign. Besides the characters above
noticed in the description of this species, it differs from
N. sulcata in sculpture, in the beaks being more promi-
nent, the hinge-line more curved, and the cartilage-pit
shorter and broader, in having more teeth on the ante-
rior side, and in the crenulations of the inside margin
extending further towards the posterior angle.
It is the Area margaritacea of Bruguiere, and Gly-
is the Nucula
cijmeris argentea of Da Costa. The fry
argentea of Brown.