Var. a. White line obsolete. Hab. Unio sub- 

 tentus, Say; Clinch river, Tennessee. 



8. . . . unicolor. Very dark, rather larger 

 than any of the preceding. Hab. Alasmodon 

 rugosus, in Clinch river; and Unio verrucosus 

 in the Holston, Tennessee. 



9. . . . reticulata. (Fig. 9.) Inferior surface 

 spotted with white, and reticulated with brown. 

 Hab. Unio viridis, Raf. 



The foregoing animals, without exception, 

 are found upon, or between the branchiae, or 

 between the branchiae and the body, of the 

 Unionidae. They appear to delight in living 

 under cover; as, although they do not pene- 

 trate into the substance of the mollusca upon 

 which they are parasitic, they will penetrate 

 into the interior of the detached pieces given 

 them for food, when kept in vessels for obser- 

 vation. Their usual size is represented by the 

 small outline beside fig. 1. The surface is 

 smooth; the legs transparent, bristled, and ter- 

 minated by hooks; the anterior pair shortest 

 and thickest, and the posterior pair longest. 



