Pvn H,i I'UitlS

434 mactrid^.
and Hanley, and (with a single exception) all the Con-
tinental zoologists, I will not prolong the discussion,
beyond remarking that Lavignonus of Ferussac (1821),
Listera of Turton (1822), Abra of Risso from Leach's
MS. (1826), and Syndosmya of Re'cluz (1846) either
have never been clearly or adequately defined, or else
are superfluous. The last-named genus has indeed been
selected by Forbes and Hanley, and separated from
Scrobicularia, without stating any particular character
by which one can be distinguished
from the other.
Clark at first considered them different, on the ground
that S. piper at a has but one gill or branchial plate on
each side, and that the palps are of a larger size in this
than in some other species.
He subsequently modified
his opinion by suggesting that the single plate might
be double : the relative magnitude of the labial palps
can only be regarded by the mere malacologist as a
matter of anv importance. The sole diversitv between
the shells called Scrobicularia and Syndosmya consists
in the former having no lateral teeth, while the latter
possesses them in one or each valve. This character
notoriously varies in species of other genera, for instance
Tellina and T)onax.
The circumstance of the tubes being separate in the
present genus, and united in Mactra and Lidraria,
might induce some conchologists to replace it among
the Tellinidcs : but the hinge is essentially Mactridan ;
and in Amphidesma, which has the same internal struc-
ture, the tubes are also disunited. I should distrust
any classification of families founded exclusively upon
the contiguity or remoteness from each other of these
outer folds or processes of the mantle called
" siphons."
' '
tubes " or