Dr. Hildreth on fresh water Shells. 285 



Length, 2.00; breadth, 2.00 ; diameter, 1.12. — Hab. Ohio, 

 Shell, compressed and deeply emarginate before ; round- 

 ed and projecting behind ; beaks flat and eroded ; ligament, 

 more elevated than the beaks, and passing between them ; 

 hinge margin, broad and strait ; anterior dorsal margin, pro- 

 jecting ; anterior margin, emarginate ; anterior basal, pro- 

 jecting ; basal margin, arcuated ; two elevated ridges, ex- 

 tending from the beaks, and projecting on the anterior dorsal 

 and basal margins, with a broad furrow between ; epider- 

 mis, dark olive ; waved transversely, and obscurely rayed 

 with green, across the waves ; cardinal teeth small, and that 

 in the right valve deeply sulcated ; lateral teeth, short and 

 thick ; posterior cicatrix, deep and smooth ; anterior one, 

 strongly impressed and rough behind ; cavity, broad and 

 shallow ; naker, white, tinged with a beautiful pea green ; 

 iridescent on the fore part. 



Re?narks.—Ha.\'mg but one specimen of this shell, I am 

 unable to determine whether it is a new variety, or only a 

 "lusus natures."* 



No. 17. Unio Alatus. — Fig. 17. 



Shell ovately triangular ; hinge margin elevated into a 

 large wing ; valves growing together en the back of the lig- 

 ament, inside purple. 



Length, 4.5 ; breadth, 6.5 ; diameter, 1.7. — Hab. Duck 

 Creek. 



Shell moderately thick, disks flat and compressed, long be- 

 fore and short behind ; beaks depressed ; ligament conceal- 

 ed between the valves ; hinge margin, very much elevated 

 and compressed ; basal margin nearly strait ; anterior dor- 

 sal, emarginate ; anterior margin, rounded and broad ; pos- 

 terior margin, rounded and narrow ; surface deeply wrinkled ; 

 teeth elevated and crenate ; anterior cicatrix, very broad ; 

 posterior composed of three distinct impressions, and also a 

 row of very small impressions across the cavity of the beaks ; 

 naker, red-purple, very briliant, and most splendidly iride- 

 scent on the forepart. 



Remarks. — It is difficult to procure a perfect specimen, of 

 a full grown subject, the wing being more or less mutilated. 

 The figure of this specimen has been drawn with great care, 



In Mr. Barnes's opinion it is new and distinct. — En. 



