Mr, Barnes on the Genera Unio and Alasmodonta, 277 

 * Shells thick and large. 



Species. 



n T?- <nn^ «• youns!;, cold. 



1. ALASMODONTA ARCUATA. 1 Ig. 20. < 7 "^ ° 1 



Shell ovate, elongated transversely, thick ; base ar- 

 cuated ; ligament elevated ; beaks depressed ; ci- 

 catrices rough. 



Hab. West Canada Creek. Mr. R. N. Havens. 



A small stream in Tappan. Mr. J. Sears. 



Cabinets of the Lyceum and Dr. Mitchill. 



My Collection. Mr. Say's Collection. 



Diam. 1.2—1.6 Length 2.1—2.6 Breadth 4.1—5.5 



Shell thick; disks convex above, and compressed below; 

 anterior side very much produced ; beaks slightly elevated; 

 ligament elevated above the beaks ; hinge margin elevated, 

 compressed, carinate ; basal margin arcuated; anterior 

 margin narrow and somewhat pointed ; posterior margin 

 rounder and broader than the anteriour ; ant. dorsal margin 

 rapidly narrowed and subtruncate ; post, dorsal impressed 

 behind the beaks ; epidermis brownish black ; surface, in 

 young specimens, smooth and glabrous, in old ones, much 

 eroded, scabrous and broken. Teeth two in the right and 

 one in the left valve, triangular, elevated and crenate ; 

 muscular impressions rough ; cavity of the beaks small ; 

 naker bluish white, on the fore parts, lightly iridescent, the 

 rest dull. Young specimens have the center of a pale 

 flesh colour, and old ones are frequently marked with ir- 

 regular greenish spots. 



Remarks. — The remarkable change in the form of this 

 species by age, as represented in the figures, might induce 

 an observer to suppose that the shells belonged to different 

 species ; but the specimens in our collections of every va- 

 riety of form, from those that are straight or even slightly 

 rounded on the base, to those that are deeply arcuated, show 

 clearly that they all belong to the same species. It is sur- 

 prising that a shell so large, and frequently occurring in our 

 waters should go long have been overlooked. This has 



