32 
pyramidal; nacre bluish white and iridescent poste- 
riorly; cavity capacious; most so under the umbonial I 
slope. 
OBSERVATIONS. 
A species distantly allied to U. iris , Lea, but dif- 
fering in its greater convexity and elevation, as well 
as in the peculiarity of its rays, which disappear on 
the anterior side; an impressed line is visible in some 
specimens, on the posterior submargin. I found 
several in the Black Warrior river, south of Blount’s 
Springs, Alabama. 
UN 10 GREEN II. 
Plate IV. — Fig. 1 
DESCRIPTION. 
Shell subovate, thick before, thinner behind; an- 
terior side not very short, rounded, beaks slightly 
prominent; umbonial slope straight, subangulated; 
epidermis wrinkled, olivaceous, rayed only on the 
posterior side; cardinal teeth thick; lateral teeth 
striated and minutely granulated; within bluish 
white, iridescent on the posterior margin; a short 
obtuse elevation passing obliquely from beneath the 
cardinal teeth; cavity moderately capacious. 
OBSERVATIONS. 
A species which cannot be confounded by an accu- 
rate observer with any other heretofore described; 
