Pvn H,i I'UitlS

MACTRA. 415
Genus II. MACTRA*, Linne. PL VIII. f. 2.
Body oval, tumid : mantle thick, open in front : tubes united,
enclosed in a sheath, of equal length, and extensile ; orifices
ciliated.
Shell triangularly oval, ventricose, nearly equilateral,
slightly gaping at the posterior end, concentrically striated,
and occasionally (but slightly) marked -with longitudinal lines :
beaks turned towards the anterior side, separate : teeth, two
thin, erect, and diverging cardinals in each valve ;
those in the
right valve are of unequal size ; and those in the left valve
equal-sized, united at their bases, and apparently forming a
single forked or chevron-shaped tooth ; laterals large and
laminar, two on each side in the right valve, and one on each
side in the left valve :
pallia! scar having a short sinus.
The shells comprised in this ancient genus are always
recognizable by their triangular shape and strong hinge-
process. Searles Wood says that, in a fossil state, they
have been obtained in the Lias and Greensand formations.
The genus is largely represented in the newer Tertiaries,
and continues in a nourishing condition at the present
day. They seem to prefer sand to mud as a dwelling-
place.
A. Shell of a plain or uniform hue ; lateral teeth and sockets
perpendicularly striated.— Spisida, Gray.
1. Mactra so'LiDA-fv, Linne'.
M. solida, Linn. Syst. Nat. p. 1126 ; F. & H. i. p. 351, pi. xxii. f. 1, 5, and
(siphons) pi.
L. f. 2.
Body more compressed than in most of its congeners, milk-
white, yellow, or light orange, occasionally tinted with brown :
mantle less fringed on the posterior side than in other parts ;
edges thickened : tubes short, but often extended three quarters
of an inch in specimens considerably under the average
size ; the orifice of each is furnished with 12-20 short pale-
yellow, brown, or reddish cirri, which spring from bulbs ;
* A kneading-trough.
t Solid.