432 Annals of the Carnegie Museum. 



the posterior distally more so; ligament moderately long, slight. 

 Long. 4; alt. 3.5; diani. 2.5 mm. Soft parts not examined. 



The young mussels are very little inflated, with the small, narrow 

 beaks projecting, the shells colorless, glassy, transparent; the right 

 cardinal is slightly curved, and its posterior end little thicker, but 

 distinctly bifid. 



Habitat: Lake Winnecook, Unity, Maine. Collected by S. Still- 

 man Berry, Aug., 1909. About forty specimens were seen, only a 

 few of them mature, or nearly so. 



Type and paratypes No. 9215, and seqq., C. M. ; paratypes in the 

 collection of Mr. S. Stillman Berry. 



This is a rather remarkable species. In spite of its commonplace 

 appearance, it is so different from others, especially in regard to the 

 formation of the cardinals, that it is difficult to assign it to any of 

 the more or less well-established groups. Li fact, the right cardinal 

 is rather like that of forms of Musciiliiim, but otherwise the mussel 

 is a true Pisidiuni. It should be noted that the configuration of the 

 hinge was the same in two specimens opened ; so it is not a case of 

 individual deformity. 



5. Pisidiuni griscolum n. sp. Mussel small, subequipartite, slightly 

 elongate, moderately inflated ; superior margin nearly straight, 

 bounded by an angle posteriorly and a more obtuse angle anteriorly; 

 supero-anterior slope somewhat marked, moderately oblique ; posterior 

 margin truncate or subtruncate at nearly right angles, inferior mod- 

 erately curved; beaks barely behind the middle, rather small, rounded, 

 or slightly flattened on top, not very prominent ; surface dull glossy, 

 with very fine, somewhat irregular striae ; color pale corneous to light 

 grayish; shell thin, translucent; hinge rather long and stout, plate 

 rather broad; cardinals short, rather well up on the plate, the right 

 angular in the middle, its posterior part thicker and grooved, left 

 both of about the same size, the anterior angular, its edge sharply 

 pointed, posterior oblique, slightly curved ; laminae somewhat massive, 

 the right anterior with its cusp about median, pointed, not abrupt, 

 posterior with cusp somewhat distal, abrupt distally, outer both rather 

 large, about half as long as their principals ; left both with cusps 

 about median, pointed, not very abrupt; ligament rather short and 

 thick. Long. 2.8; alt. 2.3; diam. 1.5 mm. 



Habitat: Lake Winnecook, Unitv, Maine. Collected bv S. Still- 



