356 A Monograph of the Species of Pisidium. 
minal, large, prominent, not approximate ; color reddish white 
when the animal is alive, but otherwise of a light transparent 
yellow; hinge margin irregular; cardinal teeth separate, of 
about the same size ; lateral teeth separate, acute, prominent. 
Dimensions. Long. 0.11; lat. 0.095 ; diam. 0.085 inches. 
GEOGRAPHICAL DISTRIBUTION. Cambridge Meadows, and 
Rowley, Essex County, Mass. (Nobis.)  Pittsfield, Mass. 
(Shurtleff. 
Ozsservations. This remarkable species, owing to its 
globose form, is not likely to be confounded with any other 
but the P. rotundatum. The latter shell, however, is less 
oblique, the margins are more rounded, the beaks are more 
central larger, and more rounded. Found very plentifully 
during the spring and early summer ; at other times of the 
year I have never been able to find it. It lives in running 
waters in company with P. variabile and P. compressum. 
f ^N 
6. Pistptum compressum Nobis. 
Cabinet of the B. S. N. H. 
Pl XI. Fig. 13, 14, 15. 
Pisidium compressum Nonis. Bost. S. N. H. Proc. iv. 164; Annals of 
the New York Lyceum of N. H., N. 219, pl. 6. 
DzscmiPrroN. Animal. Foot very long, narrow, issuing 
from the inferior opening of the mantle; syphon short. 
Shell rather small, oblique, triangular, tumid, ovate in adult, 
compressed in young, heavily striated in adult, less so !? 
young; color varying from yellow, gray, and brown, with 
zone of yellow on the margins, occasionally spotted on the 
beaks; beaks small, very prominent, very distant; hinge 
