120 cycladidje. 



PISIDIUM. Pfeiffer. 



Shell equivalve, thin, usually tumid, suboval, inequilateral, 

 smooth or concentrically striated. Hinge with one tooth 

 in the right and usually two in the left valve ; also lateral 

 teeth. Ligament external, inserted at the shorter side. 



Animal suboval, with the mantles freely open in front 

 and anteriorly, posteriorly united to form a single siphon 

 composed of the united anal and branchial tubes ; margins 

 of its orifice and of the mantle simple. Foot large, lingui- 

 form, and very extensile. 



Very small bivalves living in similar localities with Cyclas, 

 and not uncommon even in drains through meadows. Like 

 the allied genus they are ovoviviparous, and the young 

 have more compressed shells than the adult. The British 

 species of both Puidium and Cyclas have been most ably 

 investigated by the Rev. Leonard Jenyns. 



* Sides not peculiarly unequal. 

 P. obtusale, Pfeiffer (?). 



More or less swollen, very finely striolate ; umbones rather 

 projecting and very obtuse. 



Plate XXXVI. fig. 1. 



Pisidium obtusale, C. Pfeiffer, Deutsch Land und Siissw. Moll. pt. 1, p. 125, 

 pi. 5, f. 21, 22 (probably). — Jenyns, Trans. Cambridge 

 Phil. Soc. vol. iv. p. 301, pi. 20, f. ], 2, 3.— Gray, Ma- 

 nual L. and F. W. Shells, p. 282, pi. 12, f. 149.— Brown, 

 Illust. Conch. G. B. p. 94, pi. 39, f. 24. 

 Cyclas oUusalis (not of Lamarck), Nilsson, Moll. Terr, et Fluv. Suecire, p. 101 

 (probably). — Hanley, Recent Shells, vol. i. p. 90, suppl. 

 pi. 9, f. 47. 



The shape of the present species, which may be distin- 

 guished from its British congeners by its more swollen 



