Mr. Barnes on the Genera Unio and Alasmodonta. 279 
Cabinets of the Lyceum, "i Dr. Mitehill. My collection. 
Mr. Say’s collection. =~ 
Diam. .9—1.4 Length fron ae to base 3.0 Breadth 5.0 
, ength from the top of the wing 4.3—4.5 
Shell very short behind ; disks much flattened; umbones 
depressed; beaks slightly projecting ; ligament between 
the valves ; anterior lunule much compressed and folded 
across the transverse wrinkles; hinge margin eleyated into 
a large wing, straight and forming an obtuse angle with the 
post. dorsal margin ; basal margin slightly rounded, nearly 
straight; anteriour and posterior margins somewhat angula- 
ted; anteriour dorsal margin arcuated, or somewhat emar- 
ginate ; epidermis chesuut brown, glossy 3 3 surface some- 
what deeply wrinkled and striated transversely ; slightly 
elevated age and furrows diverging from the beaks to 
the anteriour margin, and distinctly impressing the inside. 
Teeth elevated, sulcated and radiating aioe the beaks; ci- 
catrices smooth ; cavity small and angular; naker bluish 
white and iridescent; surface smooth, and polished, i in old 
specimens spotted with green. 
Remarks.—This shell resembles the Unio Alatus, in the 
elevation of the wing and the connexion of the valves, and 
might at first sight be mistaken fora variety of that species ; 
but it differs in generick character, in shape, and in colour. 
** Shells thin and small. 
4. ALASMODONTA MARGINATA.—See Mr. Say’s de- 
scrip | 
5. ALASMODONTA unpuLATA.—See Mr. Say’s de- 
scription and figure. 
These two species were the only ones known when Mr. 
Say published his description. .The former of them is very 
common and assumes a great variety of forms and colours, 
Those that were brought by the N. W. Expedition are lar- 
ger than those of our eastern waters. They have the epi- 
dermis pale green, rayed ; ae are gibbous ; have ‘the beaks 
elevated, and base falcated. 
Diam. 1.0 Tength 1.4 Breadth 2.4 
