THE NAUTILUS. 48 



narrow, direct and extending to the hinge-line, the posterior tooth ia 

 separated from the interdentum by a deep groove; lateral teeth bent 

 obliquely downward from the hinge-line, two in the left valve and 

 one in the right, large and nearly straight; anterior adductor im- 

 pressions large and deep, those of the protractor-pedis well marked, 

 rather long and narrow, below and slightly behind the adductor; 

 anterior retractor impressions small and on the base of the pseudo- 

 cardinal; posterior adductor impressions large, semicircular; those of 

 the posterior retractors small, but well impressed, above that of the 

 adductor and immediately below the end of the lateral tooth; cavity 

 of the beaks shallow; nacre white. 



The female shell is thinner than that of the male and proportion- 

 ately wider, the posterior ridge being more oblique and more ex- 

 tended; the posterior line is straight or slightly curved, without the 

 emargination noted in the male; the anterior ridge is greatly pro- 

 duced beyond the basal line in a triangular prolongation and this, the 

 marsupial expansion, is of a different texture from the rest of the 

 shell, being thin and dark green as in T. capsceformis; between this 

 expansion and the posterior ridge, the base is deeply emarginate. 



Length (male) 43; height 37; diam. 22^ mm. 



Length (female) 51; height 49^; diam. '2b mm. 



Types (No. 15612 Coll. Walker), from the Holston River, Tenn. 

 (ex Lewis Coll.). Also from the Clincii River, Tenn. (Lewis); 

 Cumberland River, Port Burnside, Ky. (Wetherby) and the Holston 

 River, Knox Co., Tenn. (Andrews). 



This species, while closely related to T. foliata Hild., to which it 

 was referred by the original collectors, is clearly distinct. Foliata is 

 a much larger and heavier species and is apparently confined to the 

 Ohio and Wabash rivers, while lewisii is restricted to the Cumber- 

 land and Tennessee drainage systems, and from all the localities 

 given above is remarkably consistent in its peculiar features. Be- 

 sides being uniformly smaller, more delicate and smoother than 

 foliata, it is specially characterized by the difference in the mar- 

 supial expansion, which is triangular and comparatively narrow at 

 the extremity and of a different texture from the body of the shell. 

 In foliata, this expansion is broadly rounded and is of the same 

 texture as the remainder of the valve. 



It is named in memory of the late Dr. James Lewis, of Mohawk, 

 N. Y. (whose collection has furnished the types), who was a con- 



