BIVALVIA. 109 
two lateral teeth in each valve. Ligament external, situated on the shorter side. 
Impressions of the adductors and of the mantle indistinct. 
Animal subovate, with its mantle open on the anterior side and the margins 
without fringes, united towards the posterior, where it forms a short and single 
siphon, the orifice of which is plain ; foot large, tongue-shaped, and extensile. 
This genus has been separated from Cyc/as in consequence of a difference in their 
animal inhabitants, those of Cyclas having the siphon dichotomous, or divided near 
the end into distinct tubes, whereas in this it remains single and simple, to the 
extremity, and is not so long. The species as yet known are all small, and have 
similar habits to the preceding, frequenting ditches or pools of stagnant water, or 
where the stream is not very rapid, and like the allied genus they are perfectly 
formed in the parent animal before exclusion. They differ also in having the side 
on which the ligament is situated, the posterior, shorter than the anterior; in Cyclas 
it is the reverse. 
It has not as yet heen met with in any Deposit of an anterior date to the Newer 
Tertiaries. 
1. Pisipium amnicum, JMJidller. Tab. XI, fig. 1, a—d. 
TeLuina amNnica. Miuill. Verm. Terr. et Fluv., pt. 2, p. 205, 1774. 
— — Chem. Conch. Cab., t. vi, p. 138, t. 13, fig. 134, 1782. 
— RIVALIS. Maton. Trans. Linn. Soc., vol. ii, p. 44, pl. 13, figs. 37, 38. 
_ — Donovan. Brit. Shells, vol. ii, pl. 64, fig. 2, 1801. 
Carpium amnicuM. Moné. Test. Brit., p. 86, 1803. 
Cycias paLustris. Drap. Moll. Ter. et Fluv., p. 131, pl. 10, figs. 15, 16, 1805. 
— osLieua. Lam. Hist. des An. s. Vert, t. vy, p. 559, 1815. 
— — Brown. Ilust. Brit. Conch., pl. 17, fig. 14, 1827. 
— amnica. Turt. Brit. Biv., p. 250, pl. 11, fig. 15, 1822. 
— — Id. land and F.-W. Shells, p. 15, pl. 1, fig. 5. 
— —_ Lyell. Elem. of Geol., 2d ed., vol. i, p. 227, fig. 103, 1841. 
Pera FLUvIATILIs. Leach, MSS., fide Jenyns. 
Prsipium osiieuum. Pfeif. Land und Sussw. Moll., pt. 1, p. 124, pl. 5, figs. 19, 20, 
and pl. 1, fig. 19, 1821. 
— _ Phil. En. Moll. Sic., vol. ui, p. 31, 1844. 
— AMNICUM. Jenyns. Trans. Camb. Phil. Soc., vol. iv, p. 309, pl. 21, fig. 2. 
— _ Forb, and Hanl. Hist. Brit. Moll., vol. ii, p. 133, pl. 37, figs. 8, 9, 
and pl. O, fig. 8, 1849. 
Spec. Char. Testd ovatd, oblique trigond, ventricosdé, sulcato-striatd umbonibus 
obtusiusculis. 
Shell ovate, obliquely trigonal, ventricose, striated or sulcated, umbones rather 
obtuse. , 
Length, } an inch. 
Locality. Mam. Crag, Southwold. 
Grays, Erith (Jorris), Cropthorn (Stérickland), Clacton, Stutton. 
Faversham, and Kennet Valley (Pickering). Recent, Britain, France, Sicily. 
