A Monograph of the Species of Pisidium. 363 
P. Henslowanum Jenyns. This is one of our most com- 
mon shells, and it lives most generally in company with 
P. variabile and P. ventricosum. Found generally at the 
roots of aquatic plants. 
14. Pistprum REGULARE Nobis. 
Cabinet of the B. S. N. H. 
El, AUI. Fund 12, 13, 
Description. Shell compressed, somewhat elongated, 
tumid, rather oblique; inferior margin rounded ; anterior 
margin abrupt ; posterior margin rather sharp ; beaks neither 
large nor tumid, approximate ; striations visible ; color olive 
Steen, with a broad zone of yellow on the margins ; interior 
whitish blue ; hinge margin rounded; cardinal. teeth very 
large, widely separate, of about an equal size; lateral teeth 
well developed. 
Dimensions. Long. 0.21 ; lat. 0.15; diam. 0.11 inches. 
GrocrapuicaL pisTRiBUTION. Miami Canal, near Cincin- 
nati, Ohio, (Anthony.) : 
. Osservations. Compared to the P. variabile, this species 
Is less inflated, the beaks are smaller and less tumid, the 
Whole shell is more slight, and is moreover rendered distinct 
by its tapering form; the teeth are also more develo ed. 
Compared to the P. minus, it is less elongated; the outline 
of the margins is less rounded, and the coloring is different. 
Found in company with the P. altile, and like that species, 1t 
Seems to bury itself in the mud. 
A am 
