Pvn H,i I'UitlS

MACTRA. 419
and southern variety, and M. elliptica as the deep-water
and northern variety of one and the same species. Clark
says that M, elliptica appears distinct, because of its
delicacy and elegance ; but he adds that M. solida is at
Exmouth strictly a littoral species, and that the other is
never taken except by the dredge in the coralline zone,
six miles from the shore. Every conceivable gradation
of shape and solidity may be seen in a recent state ;
and the union of M. solida and M. elliptica is cemented
by palseontological researches— showing the advantage
(if not the necessity) of such investigations in the study
of the Mollusca. As Searles Wood has remarked, spe-
cimens of M. ovalis from the Red Crag may belong to
I may
either species.
also observe that when M. solida
gradually finds its way into deeper water than it had
been accustomed to— for instance, in descending from
the Dogger bank down its slopes to the Silver-pits on
the Yorkshire coast— the shell becomes more slender
and glossy, although of nearly the usual size. It has
then all the appearance of a large M. ellij)tica.
The typical form is the Tingonella gallina of Da Costa,
M. vulgaris of Chemnitz, and probably M. castanea of
Lamarck ; and the variety truncata is the Trigonella
zonaria of Da Costa, M. subtrwicata of Donovan (but
not of Da Costa), M. crassatella of Lamarck, and M.
crassa of Turton. The latter variety was figured by
Lister as " Concha crassa."
2. M. subtrunca'ta *, Da Costa.
Trigonella subtj-uncata, Da Costa, Brit. Conch, p. 198. M. mbtnmcato,
F. & H. i. p. 358, pi. xxi. f. 8, xxii. f. 2, and (siphons) pi. L. f. 3.
Body convex, milk-white with sometimes a faint tinge of
crenated or serrated rather
yellow : mantle having its edges
* Somewhat truncated or lopped.