BIVALVIA. 65 
The recent shell is said to be as much as a quarter of an inch in diameter: 
the largest of my fossil specimens, does not exceed the sixth of an inch in its 
extreme dimensions, measured diagonally from the umbo to the posterior part of its 
ventral margin. There is, nevertheless, but little doubt of their identity, as the 
only difference is that of size, the Crag specimens corresponding in all other charac- 
ters with the recent shell. It is very much inflated: the two valves when united, 
having a greater diameter than is given when measuring from the dorsal to the 
ventral margin; the umbo is terminal, projecting a little beyond the anterior of the 
shell, and is slightly curved; it is somewhat rhomboidal in its contour, with a little 
obliquity towards the posterior side, and a slight indentation in the ventral margin, 
the posterior side forming an obtuse angle with the edge of the shell that contains the 
ligament; the exterior is ornamented with large prominent striz, or rather small ribs 
which bifurcate, and are more numerous in the old than in the young shell, they 
are crossed and made somewhat rough by prominent and distinct lines of growth, 
at rather irregular distances, sometimes giving a decussated appearance to the shell ; 
the prominent costulated striae project beyond the margin, and give a deeply 
crenulated edge all round; the ribs are sometimes visible in the interior, although 
the specimens are often so thick as not to allow them to be seen on the inside ; and 
in that case, the impressions formed by the muscles are deeply indented, that by the 
anterior adductor is comparatively very large. The ligament seems to have been 
a strong one, as a deep linear depression is formed within the dorsal margin. This 
shell has been dredged in the living state, in 20 fathoms water, off Penzance. 
Prectuncuutus,* Lamarck, 1791. 
PrctuncuLUs PoLyLeprocineLyMus (sp.) Lisé., 1687. 
Macrra? Browne, 1756. 
Arca (spec.). Linn., 1767. 
Guycimerts. Da Costa, 1778. Humph., 1797. 
AxInzA AXINHODERMA. Poli, 1795. 
Tucreta. Bolton, 1798, sec. Herrm. 
Generic Character. Equivalve, orbicular, convex or lenticular, subequilateral, 
closed, thick and strong; externally smooth or ornamented with radiating striz or 
costa. Hinge teeth small, numerous, forming an arched or curved line, central 
denticles becoming obsolete in old shells. Ligament external, attached to a grooved 
area in each valve, with distant beaks. Impressions of the adductors two, lateral 
and strongly marked, that by the mantle entire, or without a sinus. 
* Etym. The diminutive of Pecten. 
