A Monograph of the Species of Pisidium. 359 
9. Pistp1um opscurum Nobis. 
Cabinet of Professor Adams of Amherst. 
Pisidium obscurum, Nobis. Bost. S. N. H. Proc. iv. 161. 
Description. Shell large, delicate, somewhat globose, 
elongated ; inferior margin well rounded ; anterior and poste- 
rior margins less so; beaks situated very near the posterior 
extremity, large, tumid, not approximate; surface smooth 
and polished; striations hardly visible; color dark olive 
green, lighter on the beaks, with a broad zone of yellow on 
the margins ; interior light blue. 
Dimensions. Long. 0.26; lat. 0.90 ; diam. 0.15 inches. — 
GEOGRAPHICAL DISTRIBUTION. Ohio, (Adams. 
Osservations. In outline this species is very similar to 
the P. minus, but otherwise it is different; it is more in- 
flated ; the beaks are more prominent, the color is differ- 
ent, and the shell is larger. 
10. Prsrprum aspirum Haldeman. 
Cabinet of the Academy of Natural Sciences. 
PI. XI. Fig. 24,25. Pl. SIL Fig. 1. 
Pisidium abditum Hsatpeman. Proc. Ac. Nat. Sci. 1, 53. 
Description. ^ Shell small, ovate, ventricose, somewhat 
tumid ; margins well rounded ; beaks not much removed from 
the centre, somewhat elevated and approximate; surface 
smooth, lines of growth light; color light with a broad zone 
of yellow round the margins; hinge margin rounded ; car- 
dinal teeth small, separate ; anterior tooth the larger and more 
Prominent ; lateral teeth small, acute ; teeth all strong. 
