Pvn H,i I'UitlS

274 CARDIID^.
the posterior extremity, showing that the same kind of
monstrosity occurs in different species which agree in
general structure.
In the present instance I have omitted some of the
characters, believing that a comparison
between this
species and the last would be sufficient. The two are
very similar. Such a strong family likeness runs through
all the Cockles, and indeed through every other equally
homogeneous group, that some may think it superfluous
to repeat the same particulars in describing each species.
But nothing is more difficult, and often more unsatis-
factory, than to compress the description within narrow
limits. In attempting to seize the salient points, others
which perhaps are not less material are liable to remain
" Dum brevis esse laboro, obscurus fio."
unnoticed.
More harm to science has been done bv a slight and
consequently vague diagnosis than by a too great pro-
lixity of detail.
This species may be the C. rusticum of Linne; but
there is equal probability that it is his C. tuberculatum,
which is of prior date. The latter name only is given
in his descriptive Catalogue of the Museum of Queen
Louisa Ulrica ;
and the comparison there made between
the present species and C. echinatum is full and con-
clusive. Even in the (
System a Natural/ which contains
a description of all three species, C. tuberculatum is
placed next but one to C. echinatum, and C. rusticum
follows C. edule, which is separated from the first by
eight other species.
I have observed a similar order of
sequence in the present work, as I consider it a great
help to classification, and that this method of asso-
ciating or bringing together species closely allied to
each other enables the student to distinguish them
more readily than if they were arranged indiscrimi-