34 
Rap. It is, however, more nearly allied to U. ridi~ 
hundns, of Say, and U. interruytus , of Rap.; the 
umbones are not so broad as in the latter species , 
and the posterior margin descends much more 
obliquely; none of the specimens which I found are ' 
rayed; but some are nearly destitute of the remark- 
able ridge which resembles that of U. interruytus . 
No species has the lateral teeth more beautifully 
granulated and striated. The anterior muscular 
impression is very deep, and remarkable for pene- 
trating under the cardinal tooth. 
Inhabits the Alabama river, near Claiborne, and is ■ 
rare. 
UNIO MASON I. 
Plate Y. — Fig. 2. 
DESCRIPTION. 
Shell suborbicular; disks slightly flattened; umbones 
broad, prominent, carinate behind; beaks slightly pro- 
minent; umbonial slope straight and angulated; liga- 
ment slope flattened, broad; epidermis olive, clouded 
with brown, polished; cardinal teeth prominent, com- 
pressed, very oblique; within bluish white; cayitv of 
the umbo capacious. 
OBSERVATIONS. 
A small species, which is very distinct from any 
hitherto described. I found it in the Savannah river, i 
at Augusta, in company with great numbers of £71 
congarceus, Lea, with which, perhaps, it has more 
