430 Annals of the Carnegie Museum. 



Habitat: Lake Winnecook. Unity, Maine. Collected by S. Stillman 

 Berry in 1909. There were eighteen specimens in the lot, from young 

 to mature ; in size and appearance much like compressiim mutatum, 

 with which it is associated, but the two are plainly separable. 



Type No. 9212, C. M.; paratypes in the collection of Mr. S. Still- 

 man Berry. 



2. Pisidium comprcssum mutatum n. subsp. The superior margin 

 is comparatively long and slightly curved, the supero-anterior slope 

 steep, long, straight or nearly so; the beaks are less elevate, broader, 

 with low, but sharp, ridges almost entirely surrounding a rather large 

 flattened central area on each ; surface with a somewhat silky gloss 

 produced by very fine, crowded, sharp, irregular striae; color grayish 

 to plumbeous; shell subtranslucent ; hinge stout, of the general con- 

 figuration of the species, the plate rather longer; mussel rather small. 

 Long. 3.2; alt. 3; diam. 2 mm. 



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

 man Berry in 1909. Specimens ranging from young to mature (see 

 notes under the preceding species). Type No. 9211, C. M. ; paratypes 

 in the collection of Mr. S. Stillman Berry. 



This Pisidium is markedly different from all of over a dozen named 

 and described forms and subspecies of compressmn, including those 

 seen from New England and New York. If no intermediate and 

 connecting forms are found, it should be regarded as a distinct species. 



3. Pisidium dccisum n. sp. Mussel subequipartite, short, slightly 

 oblique, well inflated, mostly so above the middle ; superior margin 

 rounded, supero-anterior slope well marked, rather steep, straight, 

 anterior end rounded angular, well below the median line, posterior 

 margin truncate at right angles, inferior well curved; beaks slightly 

 posterior, large, rounded, somewhat projecting over the hinge-margin; 

 surface polished, with very fine and slight concentric striae, some 

 spaces smooth, with very fine scratch-like lines; color corneous to 

 drab to olive, with a lighter zone along the margins, light to dark 

 brown in old specimens; shell rather thick, translucent to opaque; 

 hinge stout, rather long, plate broad, short, with the cardinals on its 

 upper part, the right not much projecting, sharply curved in the 

 middle, its anterior part short, the posterior long, grooved to bifid; 

 left anterior somewhat oblique, angular, bent upward, its free edge 

 sharply rounded or pointed, the posterior long, oblique, slightly 



