A Monograph of the Species of Pisidium. 355 
Hinge margin very much curved. . Cardinal teeth situated 
posteriorly, double, small; anterior tooth the larger; lateral 
teeth larger, acute. 
Dimensions. Long. 0.35 ; lat. 0.29; diam. 0.21 inches. _ 
GEOGRAPHICAL pisrRiBUTION.. Berkshire County ; West- 
field, Hampden County, Massachusetts. (Gould and Adams.) 
Connecticut, (Linsley.) Herkimer and Wayne Counties, New 
York, (Newcomb.) Delaware River, (Say.) New Jersey, 
(Richards.) Elk River, Cecil County, Maryland. (Halde- 
man.) Baltimore, Maryland, (Foreman.) Ohio, (Jay's Cata- 
logue.) Lake Saratoga and Lake Champlain, (Nobis.) Troy, 
NM. Y. (Newcomb.) Pittsfield, Mass. (Shurtleff.) Greenwich, 
Washington County, N. Y. (Nobis. ) Schuylerville, on Hud- 
son River, (Ingolls.) Wisconsin, (Anthony.) 
- Ossrrvations. This, our largest American species, is 
remarkable for the resemblance it bears.to the P. amnicum, 
of Jenyns, a European shell. ‘The foreign shell is however 
larger, more elongated and less oblique; the teeth are less 
prominent and not so strong. The young shell is much more 
elongated than the adult, and is of a pale white. 
Found plentifully in ponds and ditches. 
5. PisipruM ventricosum Nobis. 
Cabinet of the B. S. N. H. 
Pi. XI. Fig. 16, 17, 18. 
Pisidium ventricosum Novis, Bost. S. N. H. Proc. iv. 68. 
Descuuprioy, - Shell small, globose, tumid, rather oblique $r 
margins abrupt: striations regular and light ; beaks very ter- 
