43 



pression situated immediately beneath the tip of the 

 lamelliform teeth, in having the anterior [posterior] 

 lunule much more depressed, with slightly elevated 

 lines crossing the wrinkles."* It also differs in hav- 

 ing a purple interior, varying from an intense shade 

 of that colour, approaching an indigo blue, to a pale 

 reddish-purple. The great thickness of the superior 

 portion of the shell causes the cavity of the umbo to 

 be very shallow. 



Common in the tributaries of the Ohio, Upper 

 Mississippi, and Missouri. In Alabama, I found it 

 only in the Tennessee and its tributaries. It is a 

 common species in the mountainous region of North 

 Carolina and Virginia, where, from a poverty of cal- 

 careous matter, it is always comparatively small. 

 U. cuprea and U. atroviolacea, Raf., appear from the 

 descriptions to be mere varieties of U. dilatatus. 



UNIO TUBERCULATUS. 



Plate XXII. 

 DESCRIPTION. 



Shell subquadrangular, convex, thick; disk covered 

 with irregular tubercles, except towards the anterior 

 margin; tubercles largest near the centre of the disk; 

 anterior margin obtusely rounded; posterior margin 

 direct, emarginate; anterior lunule elliptical, profound; 

 umbo narrow, very prominent; beaks pointed, ap- 

 proximate, but generally eroded or worn in maturity; 



* Say. Amer. Conch, pi. 22. 



