116 CYPPJNID^. 



inflated, and of a light yellow color. As a general rule, the color- 

 ing of this species varies much in different localities. The speci- 

 mens collected from Rowley, Essex County, Massachusetts, are 

 larger than any 1 have seen from other places ; their color is also 

 lighter. The animal is remarkable for its want of activity. This is 

 one of our most common species, being found in nearly every 

 stream, and at all seasons of the year, though most plentifully 

 during the spring (Prme). 



Pisidium ventricosnm.* 



Pisidium ventricosum , Prime, Bost. Proc. iv. 68 (1851). 

 Musculium ventricosum, Adams, Rec. Gen. ii. 452 (1858). 

 Pisum ventricosum, Adams, loc. sup. cit. ii. 660 (1858). 



Shell small, rounded-oval, globose, ventricose, somewhat oblique, 

 slightly inequilateral, anterior side produced, posterior 

 subtruncate ; beaks small, protuberant, distant, situ- 

 ated towards the posterior side ; surface smooth, yel- 

 low ; hinge-margin curved ; cardinal teeth separate ; 

 lateral teeth short. Length, eleven hundredths of an 

 inch ; breadth, ninety-five thousandths of an inch ; 

 ' Enlarged. ' widtli, eighty-five thousandths of an inch. 



North America, in the State of Massachusetts. 



This small, globose species is not likely to be con- 

 founded with any other but P. rof.undatum, than which, 

 however, it is more oblique, the margins are more 



p. ventricosum. i i i i , • i i i 



abrupt, and the beaks more terminal and very much 

 smaller. It is very nearly allied to P. obtusale, of Europe (^Prime'). 



Family CYPEINID^E. 



Shell equivalve, globose or rounded, with a triangular outline, 

 more or less equilateral, solid, concentrically striated. or furrowed, 

 beaks twisted spirally or turned to one side ; ligament mostly ex- 

 ternal, hinge short but strong, furnished with two or three cardinal 

 teeth, besides laterals in each valve ; pallial scar entire ; muscular 

 scars oval and usually very distinct. f 



* See note, page 107. 



t In this and other cases among the Cimrhifera, where I find no description of genus or 

 family in Dr. Gould's MSS., I copy from Jeffreys. — W. G. B. 



