Mr. Barnes on the Genera Unio and Alasmodonta. 277 
* Shells thick and large. 
Species. 
1. aLAsMoponTa arcuata. Fig. 20. b y pe old. 
Shell ovate, elongated transversely, thick ; base ar- 
cuated ; ligament elevated ; beaks depressed ; ci- 
catrices rough. 
Hab. West Canada Creek. Mr. R. N. Havens. 
Asmall streamin Tappan. Mr. J, Sears. 
Cabinets of the Lyeeum 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 subiruncate ; 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. 
Remarxs.—The remarkable change in the form of this 
species by age, as represented im 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 a// belong to the same species. It is sur- 
prising that a shell so large, and frequently occurring in our 
waters should so long have been overlooked. This has 
