Pvn H,i I'UitlS

130 MYTILIM.
coast, one of them measuring nearly
two inches and
a half in length.
Chemnitz and Fabricius considered it to be a variety
of M. discors ; and Montagu noticed it as a large form
of his Mytilus discrepans. Leach, in his '
Miscellany/ excellently described it by
Zoological
the last-men-
tioned name; and he referred the Devonshire specimens
of M. discors to a small variety of the same species.
This was many years before Gray's publication; and
perhaps, in strict justice, the name of discrepans ought
to be restored and applied to the present species.
But
I will mercifully abstain from increasing the perplexity
which has so long involved the nomenclature of the
Modiolarice.
Genus III. CftENE'LLA*, Brown. PI. III. f. 4.
Body roundish-oval : mantle open in front, and folded behind
into a sessile excurrent tube : foot worm-shaped, the
point being disk-like and issuing
out of a sheath.
Shell oval or rhomboidal, cancellated by longitudinal ribs
and transverse plates : beaks straight : ligament small : hinge
of each valve furnished with an upright tooth, which is crenu-
lated as well as the hinge-plate.
The name of this genus was probably derived from
the circumstance of the hinge-plate being slightly
notched. The shell is entirely composed of nacre, and
has a silvery hue when deprived of the epidermis : in
this respect it resembles that of Nucula. The animal is
unlike that of Modiolaria. It has the mantle completely
open in front, instead of being folded into a special tube
for the entry of food and water ; the excretory tube is
exceedingly short, and sessile, instead of being produced
* A little notch.