Pvn H,i I'UitlS

126 MYTILIDJE.
shaped ; the central area is depressed ; and the beaks
are placed at a greater distance from the anterior
margin. Mr. Dennis says that it is frequently imbedded
in a very snug little nest on sea- weeds, but never among
their roots, where M. discors takes up its abode and
occurs in great numbers. My largest specimen
is half
an inch long; but I believe it is usually very much
smaller, and that in cubical contents it is not a tenth of
the last species.
3. M. dis'cors*, Linne.
Mytilus discors, Linn. Syst. Nat. p. 1159. Crenella discors, F. &H. ii.
p. 195, pi. xlv. f. 5, 6 ; and pi. xlviii. f. 5.
Body yellowish-white, with a few scattered minute specks
of flake-white : incurrent tube formed by a wide protuberant
slit of the mantle in front for the admission of food and
water: excurrent tube conical, and projecting considerably
beyond the shell at the posterior or broader end : foot strapshaped,
capable of being extended more than twice the
length of the shell, and used (like that of Mytilus edulis) for
crawling.
Shell oval, much broader than either of the other species
above described, somewhat compressed, rather thin, usually
not glossy : sculpture, 10-12 remote ribs on the anterior side,
and 30-40 close-set ribs on the other side ; middle area de-
pressed and smooth ; transverse stria? very minute and fine :
colour yellowish-brown : epidermis rather thick, light green :
margins rounded on all sides, except the ventral, which is
nearly straight : beaks small, rather prominent, incurved and
diverging, placed near the anterior margin :
byssal sinus,
ligament, and hinge-line as in the other species : hinge-plate
strengthened by an internal rib, and obliquely and strongly
toothed ; ligamental groove narrow : hinge slightly reflected
and thickened, crenulated: inside highly nacreous, purplishyellow
or liver- colour, with the anterior and posterior edges
finely
notched : scars rather distinct. L. 0-5. B. 0-35.
Var. 1. angustior. Shell smaller, narrower, and more glossy.
* Disagreeing.