80 THE NAUTILUS. 



beaks ratlier posterior, projecting over the hinge margin, slightly 

 bulging anteriorly and posteriorly, flattened laterally ; surface with 

 very tine, almost regular strife and a few faint lines of growth, with 

 a slight, silky gloss ; color light to brownish-horn ; shell scarcely 

 translucent ; nacre thin, with the muscle insertions visible but not 

 impressed ; hinge slight to moderately stout, plate rather narrow, 

 right cardinal tooth curved, occupying the whole width of the plate, 

 its anterior end abruptly thick and grooved or bifid, its ends are con- 

 nected by the sharp, inferior edge of the plate, thus forming a groove ; 

 left anterior cardinal tooth short, curved or angular, the posterior 

 quite short, small, oblique ; lateral teeth moderately stout, with short, 

 pointed cusps, the outer ones of the right valve quite small but dis- 

 tinct ; ligament short, comparatively stout. 



Long. 3.2, alt. 2.8, diam. 1.8 mill.; long. 2.7, alt. 2.4, diam. 1.6 

 mill. 



Habitat: Little Black Creek and lakes in Muskegon Co., Michi- 

 gan, sent by Mr. Bryant Walker. 



This species is ditTerent and distinct from all our Pisidia. It 

 somewhat resembles P. ultramontanum Pr., but is much smaller, its 

 beaks are narrower, of different shape and more prominent. Some 

 of the specimens from the creek are higher over the beaks and in the 

 posterior part, and thus the mussel is of a rather different shape. In 

 others, the beaks are less flattened, laterally. 



Pis. ro^oelli n. sp. Mussel well inflated, elliptical-ovate in outline, 

 angles at scutum and scutellum slightly projecting, broadly rounded; 

 posterior margin just perceptibly subtruncate, supero-anterior slope 

 slightly marked ; beaks a little posterior (" in normal position "), 

 large, rounded, projecting over the hinge margin ; surface shining, 

 slightly and irregularly striate, with a few coarser lines of growth ; 

 horn colored to brownish over the beaks, usually with a lighter zone 

 along the margins, not sharply defined ; shell translucent, rather 

 thin ; hinge slight, plate rather narrow ; right cardinal tooth angular, 

 with its posterior part thicker and grooved, left anterior short, tri- 

 angular, placed high up on the plate, the posterior much longer, 

 oblique, curved ; lateral teeth with rather short, abrupt cusps in the 

 left valve ; the outer ones in the right valve small ; ligament slight. 



Long. 7.5, alt. 6.2, diam. 4.5 mill. 



Habitat: Near Sisson, at the foot of Mount Shasta, California, 

 collected by Rev. J. Rovvell. 



