366 A Monograph of the Species of Pisidium. 
OssEnvaTrows. This is a truly remarkable species, and, 
owing to its ridges or sulcations, is not likely to be con- - 
founded with any other. Compared to the P. abditum, the 
outline is less spherical and more elongated. Compared to 
the P. minus, it is more inflated, less elongated, broader, and 
not only more sulcated but also larger. 
"SN 
18. PISIDIUM EQUILATERALE Nobis. 
Cabinet of the B. S. N. H. 
Pl. XII. Fig. 23, 24, 25. 
Descrirtion. Shell small, stout, heavy, somewhat in- 
flated, rhomboidal ; margins of very nearly the same length ; 
posterior margin abrupt ; inferior and anterior margins slightly 
rounded ; beaks large, very prominent, rounded, not approxi- 
mate ; color varying from a light brown to a yellowish brown; 
in some cases there is a zone of light yellow on the inferior 
margin, and in others there are two or three zones; lines of 
growth very fine. The epidermis is in some specimens 
glossy; hinge margin lightly curved ; cardinal teeth small, 
lateral teeth rudimentary. 
Dimensions. Long. 0.15; lat. 0.14 ; diam. 0.10 inches. 
GrocnaPHiCAL pisrRiBUTION. Augusta, Me. (Nobis) 
Groton, Mass. (Lewis. Hudson River, (Ingolls.) Herki- 
mer County, N. Y. (Lewis.) 
Osservartons. This shell is remarkable for its short and 
quadrangular form, giving it somewhat the appearance of @ 
Cyclas. Compared to the P. variabile it not only differs in 
not being oblique, but also in size. Found in the Spring 
