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 

 undatus, 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. 



UNIO VIBEX. 



Plate iV.—Fig. 3. 



description. 



Shell elhptical, 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 brown; and with green interrupted 

 rays, extending beyond the middle, but obsolete on 

 the anterior margin; cardinal teeth direct, prominent, 



