10 



the summits with numerous, slender, slightly elevated lines 

 bending in different directions. 



This is evidently a young shell, but we know nothing of the 

 adult. The teeth are more like those of the undulata, Nob. 

 than any other species, but the form and sculptured anterior 

 margin amply distinguish it. 



It was brought from Virginia by Mr. I. Lukens, and by him 

 presented to me. 



A. edentula. Shell transversely oblong-oval, somewhat com- 

 pressed, dark olivaceous or brownish, obsoletely radiated (in 

 the young:) nates behind the middle but remote from the pos- 

 terior edge, but little prominent, undulated: anterior margin 

 not remarkably compressed, and not radiated: within white, 

 margined with opake white more or less distinct, but sometimes 

 obsolete: teeth obsolete, but the peculiar curvature beneath 

 the beaks shows their locality. 



Length, one inch and seven-tenths; breadth, three inches. 

 Convexity, one inch. Inhabits Wabash River. 



Approaches nearest to A. marginata Nob.; but the want of 

 teeth, and the simple anterior margin, at once distinguish it. 



C Y C L A S . 



0. edentula. Shell transversely oval, inaequilatcral, with 

 somewhat elevated and regular transverse lines: beak not ele- 

 vated above the general surface; cardinal tooth very small, 

 linco'or, oblique and not elevated higher than the edge of the 

 hinge margin: colour brown: umbones decorticated. 



Length, seven-twentieths of an inch. Breadth, two-fifths 

 of an inch. Inhabits Canandaigua Lake. 



This is distinguishable by the diminutive teeth, which are 

 not visible in a profile view of the hinge. 



C. transversa. Transversely oblong subovate, subinsequilate- 

 ral: anterior margin decidedly more widely rounded than the 

 posterior margin: beak obviously elevated above the general 

 curvature: cardinal teeth double, distinct. 



Length, over one-fourth of an inch; breadth, nine-twentieths 

 of an inch. Inhabits Kentucky. 



This shell is more transverse than even the similis. Nob. 



