108 MOLLUSCA FROM THE CRAG. 



Shell transverse, ovate, tumid, very thin and fragile, slightly inequilateral, posterior 

 side the larger, and subquadrangular ; anterior rounded, very finely striated con- 

 centrically ; ventral margin lightly curved. 



Length, \ an inch. Height, -fths of an inch. 



Locality. Mam. Crag, Bulcham, Southwold. 



Cropthorn, Grays, Clacton, Stutton, Faversham. 



Recent, Britain, France, Sicily. 



This species is abundant in the Fresh-water deposits of Clacton and Stutton. The 

 principal form is nearly ovate, and some have both sides equally rounded, but occa- 

 sionally the posterior is obtusely angulated, the line of hinge carrying the lateral tooth 

 rather further backwards than ordinary, giving it on that side a somewhat square 

 outline. The animal in the recent state has strong adductor muscles, but they have 

 made only a very indistinct impression upon the shell ; the hinge is furnished with an 

 apparatus well adapted to assist the close security of the valves, the right one having 

 a sharp prominent arched or diverging tooth, immediately beneath the umbo, which 

 locks into or between two others of similar form in the left valve ; the lateral teeth 

 are prominent and at unequal distances, that on the posterior side being considerably 

 the more distant ; in the left valve there is one before and one behind the umbo 

 and in the right there are two on each side, between which the single one is inserted 

 when the valves are closed. Some specimens are quite smooth, while others are 

 distinctly marked with numerous, obtuse concentric ridges. In the young state the 

 shell is much flatter than when full grown. 



This species is common throughout Europe, extending its range from Sicily on 

 the south to Sweden in the North. 



A few specimens of this fragile shell were found by Capt. Alexander, in the 

 Estuary portion of the Mammaliferous Crag. 



Pisidium,* Pfeiffer. 1821. 



Tellina (sp.). Linn. 



Cardium (sp.). Poli. 



Cyclas (sp.). Lam. 



Pisum. Megerle, 1811, fide Gray. 



Pera. Leach, MSS., 1819. 



Euglesia. Id. „ 1820. 



Galileja. Costa, 1839, fide Phil. 



Generic Character. Shell small, equivalve, inequilateral, subovate, more or less 

 inflated, somewhat thin ; in the recent state subpellucid, and covered with an 

 epidermis ; smooth or concentrically striated. Hinge with one or two cardinal, and 



* Etym. Pisum ? 



