A Monograph of the Species of Pisidium. 357 
margin angular; cardinal teeth situated centrally, double, 
small; lateral teeth elongated. 
Dimensions. Long. 0.16 ; lat. 0.14; diam. 0.9 inches. 
GEOGRAPHICAL DISTRIBUTION. Cambridge Meadows, Mass. 
(Stimpson.) Augusta, Me. (Nobis.) Connecticut River, 
Mass. (Nobis.) Groton, Mass. (Lewis.) Lake Champlain, 
Lake Saratoga, (Nobis.) Greenwich, Washington County, 
N. Y. (Nobis.) Herkimer County, N. Y. (Lewis.) 
OssERvATIONs. This species bears much resemblance to 
the P.altile Anthony. It is, however, more inflated, less 
oblique, less tumid; the beaks are smaller, the apex of the 
shell is not so elevated, the longitudinal length is greater, and 
the hinge margin is not quite so angular. Compared to the 
P. variabile, it is less inflated, smaller, more oblique, and of a 
different color. 
The animal is remarkable for its liveliness. Found spa- 
ringly during the spring, and not at all in winter. It inhabits 
both running and still water, and buries itself somewhat in 
the mud 
f^ 
7. Pisiprum rotunpatum Nobis. 
Cabinet of the B. S. N. H. 
Pl. XI. Fig. 19, 20, 21. 
Pisidium rotundatum Nosis. Bost. S. N. H. Proc. iv. 164. 
Descriprion. Shell small, globose, ventricose, tumid ; 
anterior and inferior margins rounded ; posterior margins 
Somewhat abrupt; beaks very large, prominent, rounded, 
Approximate, very little removed from the centre; surface 
; striations very light; color yellow, darker on the 
