369 A Monograph of the Species of Pisidium. 
known in the country. That it is not an immature shell, I 
have satisfied myself from having found the young within it. 
Compared to the P. minus, it not only differs in size but also 
in being more equilateral, and having the beaks obtuse on the 
summits. Compared to the P. tenellum, it is less globose, 
more equilateral, and the beaks are neither so swollen nor as 
rounded. 
13. PisrpiuM rerrucineum Nobis. 
Cabinet of the B. S. N. H. 
Pl. XII. Fig. 8, 9, 10. 
Pisidium ferrugineum Nobis. Bost. S. N. H. Proc. iv. 162. 
Description. Animal. Foot very long and narrow, issu- 
ing from the anterior extremity of the shell. Syphon ex- 
tremely short. 
Shell small, ovate, somewhat globose ; margins well round- 
ed, somewhat equilateral; beaks very small, but tumid, very 
distant; surface smooth, lines of growth minute, but regular 
and well visible; color pale yellow ; hinge margin curved ; 
cardinal teeth large, separate ; anterior tooth the more promi- 
nent ; lateral teeth prominent. 
Dimensions. Long. 0.17 ; lat. 0.13 ; diam. 0.11 inches. 
GEOGRAPHICAL DISTRIBUTION. Cambridge Meadows, Mass. 
(Stimpson.) Rowley, Essex County, Mass. (Nobis.) Gro- 
ton, Mass. (Lewis.) Herkimer County, N. Y. (Lewis) 
Oxsservations. This species is remarkable for the eleva- 
tion of its beaks, which stand forth on its back like tubercles 
or excrescences. It differs from P. minus, in being smaller, 
more convex, and not so elongated, as also in being more 
equilateral. 'The beaks are generally covered with some 
dark ferrugineous substance. The foreign analogue is 
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