206 MOLLUSCA FROM THE CRAG. 
Shell ovately oblong, or rather subtrapezoidal, thick, and strong; posterior side 
the longer and broader, somewhat angulated ; ornamented with transverse lamelle, 
radiatingly striated between the interstices; three teeth in each valve; palleal sinus 
large and subangulated. 
Length, 3ths of an inch; height, ths ditto. 
Locality. Red Crag, Walton-on-the-Naze. 
Recent, Mediterranean, Britain. 
Only two specimens of this species are in my cabinet, and they are all that I 
have seen; these are somewhat rubbed, with the lamellee much worn down, but quite 
perfect enough to justify their being considered identical ; my specimens are rather 
shorter than the generality of recent shells, although I believe in those there is con- 
siderable variation, and | have assigned it to the living species without any hesitation. 
In the recent state it is found imbedded in sponges and seaweed, as well as in rocks, 
and ranges from low-water mark to ten or a dozen fathoms. 
CyTHEREA.* Lamarck, 1805. 
ActrnosuLus. Klein, 1753. 
Cuama. Browne, 1756. 
Venus (sp.) Linn. 
CALListTaA CALLISTODERMA (sp.) Poli, 1795. 
Hystrroconcna. Lang., 1722. 
Meretrix. Lam., 1799. 
CuHIonE. Gray, 1838. 
DIONE. Id. 1848. 
GrussLya (sp.) Agass., 1845. 
Gen. Char, Shell equivalve, inequilateral, closed, usually more or less transversely 
oblong or ovate, sometimes obtusely triangular, generally thick and strong, with 
slightly prominent umbones. Surface sometimes transversely sulcated, usually smooth. 
Hinge furnished with three teeth in one valve, and four in the other; anterior one 
diverging under the lunule. Muscular impressions large and ovate, that by the 
mantle with a sinus generally large and wide. Ligament external. 
Animal of the form of the shell, with the mantle freely open, having the margins 
without fringes ; siphons long, united nearly to their extremities ; orifices surrounded 
with tentacles. Foot large and tongue-shaped, not furnished with a byssal groove. 
This genus may be considered as being distinguished more especially by its 
peculiarity of hinge. The anterior tooth, which gives a prominent character to the 
genus, is placed at right angles to the more cardinal teeth, and is situated at a little 
distance immediately under the lunule, giving great firmness and security to the close 
union of the valves; the composition of the shell is also somewhat different from that 
* Etym., one of the names of Venus. 
