364 A Monograph of the Species of Pisidium. 
15. Pisrprum arcuatum Nobis. (Fossil.) 
Cabinet of the B. S. N. H. 
Pl. XHI. Fig. 14, 15, 16. 
Description. Shell small, rather elongated, somewhat 
depressed ; anterior margin somewhat angular; other mar- 
gins well rounded ; beaks terminal, not tumid, well rounded, 
approximate ; lines of growth numerous, rather coarse, epi- 
dermis light ; color deadish yellow; hinge margin nearly on 
a straight line; cardinal teeth small, double; lateral teeth 
formed by the projection of the anterior cardinal tooth. 
Dimensions. Long. 0.15; lat. 0.19; diam. 0.11 inches. 
GEOGRAPHICAL DISTRIBUTION. Brattleborough, Vt. (Ingolls.) 
OssERvaTIONS. This species somewhat resembles the P. 
variabile, but it is less inflated, less oblique, and the beaks 
are not as prominent. The specimens that I have seen 
retained, most of them, their epidermis; and in some the 
enamel of the interior was still visible. Found by Dr. In- 
golls in company with two fossil species of Cyclas, which 
appear likewise to be new. 
dies 
16. Pisip1um zonatum Nobis. 
Cabinet of the B. S. N. H. 
Pl. XII. Fig. 17, 18, 19. 
Pisidium zonatum Nobis. Bost. S. N. H. Proc. iv. 162. 
Description. Shell large, moderately inflated, transpa- 
rent, somewhat elongated ; margins all well rounded ; beaks 
large, not prominent, well rounded, approximate ; lines o 
