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 Cyclas 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 been met with in any Deposit of an anterior date to the Newer 

 Tertiaries. 



1. Pisidium amnicum, Mutter. Tab. XI, fig. 1, a — b. 



Tellina amnica.. Mull. Verm. Terr, et Fluv., pt. 2, p. 205, 1774. 



_ _ Chem. Conch. Cab., t. vi, p. 138, t. 13, fig. 134, 1782. 



— rivaiis. Maton. Trans. Linn, Soc, vol. iii, p. 44, pi. 13, figs. 37, 38. 



— — Donovan. Brit. Shells, vol. ii, pi. 64, fig. 2, 1801. 

 Cardium amnicum. Mont. Test. Brit., p. 86, 1803. 



Cyclas palustris. Drap. Moll. Ter. et Fluv., p. 131, pi. 10, figs. 15, 16, 1805. 



— obliqda. Lam. Hist, des An. s. Vert, t. v, p. 559, 1815. 



— — Brown. Illust. Brit. Conch., pi. 17, fig. 14, 1827. 



— amnica. Turt. Brit. Biv., p. 250, pi. 11, fig. 15, 1822. 



— — Id. Land and F.-W. Shells, p. 15, pi. 1, fig. 5. 



— — Lyell. Elem. of GeoL, 2d ed., vol. i, p. 227, fig. 103, 1841. 

 Pera fluviatilis. Leach, MSS., fide Jenyns. 



Pisidium obliqdum. Pfeif. Land und Sussw. Moll., pt. 1, p. 124, pi. 5, figs. 19,20, 



and pi. 1, fig. 19, 1821. 

 _ _ Phil. En. Moll. Sic, vol. ii, p. 31, 1844. 



— amnicum. Jenyns. Trans. Camb. Phil. Soc, vol. iv, p. 309, pi. 21, fig. 2. 



— — Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 133, pi. 37, figs. 8, 9, 



and pi. O, fig. 8, 1849. 



Spec. Char. Testa ovatd, oblique triyond, venlricosd, sulcato- striata umbonibm 

 obtusiusculis. 



Shell ovate, obliquely trigonal, ventricose, striated or sulcated, umbones rather 

 obtuse. 



Lenyth, \ an inch. 



Locality. Mam. Crag, Southwold. 



Grays, Erith [Morris), Cropthorn {Strickland), Clacton, Stutton. 

 Faversham, and Kennet Valley {Pickeriny). Recent, Britain, France, Sicily. 



