354 A Monograph of the Species of Pisidium. 
separate ; surface polished ; lines of growth regular, visible ; 
color dark olive-brown, shade lighter on the margins and on 
the beaks; interior light blue; hinge margin very angular ; 
cardinal teeth small, separate ; lateral teeth prominent. 
Dimensions. Long. 0.07; lat. 0.07; diam. 0.09 inches. 
GEOGRAPHICAL DISTRIBUTION. Canandaigua Lake, N. Y. 
(Nobis.) Miami Canal, near Cincinnati, Ohio, (Anthony.) 
Oxservations. This shell, by its oblique and wedge-like 
form, is entirely different from any other known species. 
Compared to the P. dubium, it is less elongated ; the inferior 
margin is shorter, and the beaks are more tumid. Compared 
to the P. Adamsii it is more oblique, smaller and less full. 
The P. variabile is more inflated and larger. 
Found in company with the P. regulare Nobis. It seems 
to bury itself in the mud. 
an 
4. Pistprom pusium Gould. 
Cabinet of the B. S. N. H. 
Pl. XI. Fig. 4, 5, 6. 
Cyclas dubia Say; Nichol, Encyclo, Amer. Edit. iv. pl. 1, fig. wf 
Gould’s Report, 75, fig. 56; De Kay, 293, pl. 25, fig. 261. 
Pisidium abruptum HarpEMAN ; Proc. Ac. Nat. Sci. 1, 53. 
Pisidium dubium Gouto. Agassiz's Lake Superior, 245. 
Dzscmrriow. “Shell rather thick, triangular, with its 
corners rounded ; beaks at one end very little elevated ; base 
and longer side regularly rounded; valves tumid; surface 
with minute concentric wrinkles, which towards the 
enlarge with obvious folds; color a light olive with darker 
zones and a marginal border of yellowish ; within bluish or 
greenish." (Gould's Report, 75.) Fm din 
