31 
posterior margin obliquely subtruncated; cardinal 
teeth nearly transverse, subcompressed; regularly 
striated; posterior muscular impressions distinct, 
slightly impressed; nacre very white, iridescent. 
OBSERVATIONS. 
This species is allied to triangularis, of Raf. and 
midatus, of Barnes; but is more compressed than 
either; it never assumes the obliquely elongated form 
of the first, but is more oblique than the latter, and 
the substance of the shell is not so thick; the rays are 
broad and interrupted, and are only visible on the 
umbones of old specimens. Of many specimens of 
undatus, which I found in Alabama, not one was 
rayed; this species is never destitute of rays; the 
epidermis is profoundly wrinkled. 
I obtained the species only on the banks of Elk and 
Flint rivers, tributaries of the Tennessee. 
BNIO VIBEX. 
Plate IV. — Fig. 3, 
DESCRIPTION. 
Shell elliptical, ventricose, rather thin; posterior 
margin wide, very oblique and arcuated; end margin 
obtusely rounded; ligament margin slightly elevated; 
umbonial slope rounded; epidermis yellowish olive, 
clouded with browm; and with green interrupted 
rays, extending beyond the middle, but obsolete on 
the anterior margin; cardinal teeth direct, prominent, 
