THE NAUTILUS. 19 



nearly so, in others obsolete; beaks rather broad, rounded or slightly 

 flattened, little prominent over the upper margin; surface with very 

 fine, slight, irregular strise, shining; color pale horn to light amber, 

 shell translucent to transparent; hinge very slight, long, curved, 

 plate narrow; cardinal teeth: the right rather long, straight or 

 slightly curved, longitudinal, its posterior end more elevated, slightly 

 thicker and projecting downward over the hinge plate, the left ones 

 rather long, nearly longitudinal and parallel, lamellar, the posterior 

 (upper) slightly curved; "lateral teeth": the posterior much longer 

 than the anterior, slight, curved, the right not raised in a cusp, the 

 left with a slight cusp, the right anterior with a rounded, the left with 

 a sharp and abrupt cusp, outer ones in the right valve wanting, or a 

 mere trace of the anterior; ligament slight. 



Long. 2. 5, alt. 2.0, diam. 1.4 m., the largest; average: 2.3 m. long. 



Habitat: Elk Rapids, Michigan. About 300 specimens were seen, 

 at all stages of growth, most of them dead and bleached shells, sent 

 for examination by Mr. F. C. Baker. Types are in the collection 

 of the Chicago Acad. Sc. and that of the Carnegie Museum. 



In shape and appearance, this Pisidium resembles a form of tenw's- 

 simum St., but is much smaller and slighter, and the formation of 

 the hinge is radically different, as ascertained on quite a number of 

 specimens of both species. In fact, the " lateral teeth " are quite 

 different from those of all other Pisidia seen. 



PISIDIUM SUBROTUNDUM, n. sp. Mussel small, moderately and 

 regularly to rather well inflated ; outlines oval to short oblong to 

 nearly circular; superior margin straight or nearly so, as especially 

 noticeable in younger specimens ; beaks slightly posterior, rather 

 broad (in most forms), moderately projecting over the hinge margin, 

 thus completing the round outlines, except for the slightly project- 

 ing angles at the scutum and scutellum ; surface with very fine, sub- 

 regular to irregular striae and usually a few well marked lines of 

 growth, dullish to somewhat shining; color horn or light amber to 

 straw to whitish ; shell translucent (in the horn colored) to opaque 

 (in the light colored specimens), thin ; hinge slight and teeth small ; 

 ligament rather short and slight. 



Long. 3.2, alt. 2.7, diam. 1.8 (100: 84.4 : ,56.3) average. 



Long. 3.6, alt. 3, diam. 2.3 (100 : 83.3 : 63.9) Albany riv. 



Long. 2.8, alt. 2.5, diam. 1.8 (100 : 89.3 : 64.3) Isle Royale. 



Hab.: New England, Anticosti Is., to Michigan. Jupiter river, 



