206 MOLLUSCA FROM THE CRAG. 



Shell ovately oblong, or rather sub trapezoidal, thick, and strong ; posterior side 

 the longer and broader, somewhat angulated ; ornamented with transverse lamellae, 

 radiatingly striated between the interstices ; three teeth in each valve ; palleal sinus 

 large and subangulated. 



Length, fths 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 lamellae 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 I 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. 



ACTINOBULUS. Klein, 1753. 



CHAMA. Browne, 1756. 



VENUS (sp.) Linn. 



CALLISTA CALLISTODERMA (sp.) Poll, 1795. 



HYSTEKOCONCHA. Lang., 1722. 



MERETRIX. Lam., 1799. 



CHIONE. Gray, 1838. 



DIONE. Id. 1848. 



GKESSLYA (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. 



