PISIDIUM. 



109 



Genus PISIDIUWI, Pfeiffer. 1821. 



Shell sub-oval, incquipartitc, teeth small, one on the right and 

 Wo on the left valve, marginal teeth long, two in the right valve. 

 Mantle of animal open in front, uniting behind to form a short 

 simple siphon ; foot vermiform, expanding into a disk on which to 

 crawl. 



Pisidmm dubium. 



Fig. 56. 



Shell oblique, triangular-ovate, pale-olivaceous, with fine concentric ridges; 

 beaks but slightly elevated ; teeth strong, white. 



Cyclas dubia. Sat, Nicholson's Encyc. Amer. ed. iv. pi. 1, fig. 10 (1816). — Gould, Inv. 



Mass. 75, fig. 56. — De Kay, Nat. Hist. New York, 293, pi. 25, fig. 261. 

 Pisidiiiin dubium, Gould, in Agassiz's Lake Superior, 245 (1850). — Prime, Bost. 



Journ. vi. 35-1 (1852), pi. 11, figs. 4, 5, 6 — Chexu, Man. de Conch, ii. 105, fig. 478. 

 Pisidium abruptum, Haldemax, Proc. Acad. Nat. Sc. i. 53. 



Shell small, rather thick, triangular, with its corners rounded, 

 shining ; beaks at one end, very little elevated ; hinge margin and 

 short end straight, the two lines forming a right an- 

 gle ; base and the longer side regularly rounded ; 

 valves tumid, surface with minute, concentric wrin- 

 kles, which, towards the base, enlarge into obvious 

 sharp folds ; color a light olive, with darker zones, 

 and a marginal border of yellowish ; within l)luish 

 or greenish ; teeth well developed. Length, one 

 fourth of an inch ; height, one fifth of an inch ; breadth, three 

 twentieths of an inch. 



Found in streams in Bristol County, and in ditches in the Cam- 

 bridge meadows, in company with the last species (Cyclas eleg-ans), 

 and probably in all parts of the State. Lakes about Halifax ( Wil- 

 lis} ; Ottawa, Montreal, Anticosti, and all Lower Canada (Bell) ; 

 Connecticut (Linsley) ; Maryland (Foreman) ; New York (New- 

 comb, Prime, Lig-alls) ; Wisconsin (Anthony). 



Here, again, we have a shell bearing a close 

 resemblance to a Transatlantic species, the C. 

 obliqua of Lamarck, C. amnica of some writers. 

 The foreign shell, however, is somewhat longer, 

 less inequipartite, and more oval, and the wrin- 

 kles are somewhat more conspicuous. The young shells of our 

 species have the beaks more removed towards the centre, and the 



p. r/iibiinn. 

 Enlarged. 



Fig. 416. 



P. dubium. 



