Pvn H,i I'UitlS

CARDIUM. 287
umbones central : lunide slight and indistinct : ligament short
but strong, horncolour, with sometimes a greenish tint, continued
on the anterior side in : large specimens hinge-line
curved :
gently
hinge-plate thick and rather broad, reflected
on the posterior side, where it is usually stained with chocolate
colour, occupying about one-third of the circumference : teeth,
in the right valve one double cardinal, shaped like a reversed
Y, and set obliquely, and a strong but short double lateral on
each side ; inner teeth and laniinse much smaller and more
sunk than those next to the hinge-line ; in the left valve are
similar cardinal and lateral teeth, having their relative size and
position reversed, but these laterals are not so disproportionately
small as those in the right valve : inside somewhat
glossy and nacreous, deeply stained with purplish-brown on
the posterior side, fluted towards the margin, which is armed
with short spikes, corresponding with the outside furrows :
palUal scar slight and wavy :
terior one the longer. L. 1*6. B. 1*7.
muscular scars very strong, pos-
Yar. 1. rustica. Shell smaller and thinner, with the posterior
side usually more produced and wedge-shaped : colour
purplish-brown or light orange, often disposed in transverse
rows. C. rusticum, Chemn. Conch. Cab. vi. p. 201 (referring to
Linne's species of the same name), pi. 19. f. 197.
Yar. 2. cremdata. Shell oval and more equilateral ; front
margin nearly straight. C. crenulatum, Lam. An. sans Yert. vi.
p. 13.
Habitat : Gregarious everywhere in our sandy bays,
from low-water mark to a few fathoms. Var. 1 . Estuaries
of brackish water. Var. 2. Plymouth (Reeve) ; Herm
(Lukis); Dogger Bank (Rich). This well-known spe-
cies occurs in all our pliocene tertiaries from the latest
raised beach to the Coralline Crag ;
and I have found
it in upper miocene strata near Antibes. It is very
generally distributed through the North Atlantic Ocean,
from Iceland (Mohr) to the iEgean (Forbes). The
variety rustica lias also a range of similar extent ; and,
according to MidclendorfF, it inhabits the Aral Sea,
Caspian, Black Sea, and Russian Lapland. Lamarck
gives " La Manche " for the other variety ; and I am