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, 4 an inch. Height, 3ths 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. 
PisiptuM,* Pfeifer. 1821. 
TELLINA (sp.). Linn. 
CarpivuM (sp.). Poli. 
Cyctas (sp.). Lam. 
Pisum. Megerle, 1811, fide Gray. 
Pera. Leach, MSS., 1819. 
Euetesra. 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 ? 
