﻿TRUNCILLA 21 



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 pro- 

 portionately wider, the posterior ridge being more oblique and 

 more extended ; the posterior line is straight or slightly curved, 

 without the emargination noted in the male ; the anterior ridge 

 is greatly produced beyond the basal line in a triangular pro- 

 longation and this, the marsupial expansion, is of a different 

 texture from the rest of the shell, being thin and dark green 

 as in T. capsccformis: between this expansion and the posterior 

 ridge, the base is deeply emarginate. 



Length (male) 43. height 37, diam. 22.5 mm. 



Length (female) 51. height 49.5. diam. 25 mm." (Walker.) 



Type locality, Holston River, Tenn. Also, Clinch River and 

 Holston River, Knox Co., Tenn., and Cumberland River, Port 

 Rurnside, Ky. 



Truncilla Icwisii Walker, Naut. XXI\\ iqio, p. 42, pi. iii, 



fi.2:s. 3.- 4. 5- 



"This species, while closely related to '/. foliata Hild., is 

 clearly distinct. Besides being uniformly smaller, more del- 

 icate and smoother than foliata, it is specially characterized by 

 the difference in the marsupial expansion, which is triangular 

 and comparatively narrow at the extremity and of a different 

 texture from the body of the shell. Tn foliata, this expansion 

 is broadly rounded and is of the same texture as the remainder 

 of the valve." 



Truncilla stewardsoni (Lea). 



Shell irregularly rhomboid, solid, more or less inflated, sub- 

 equilateral ; beaks not greatly elevatetl, the umbonal region 

 elongated and sometimes slightly compressed ; posterior ridge 

 well developed, narrowly rounded or biangulate, in front of 

 which is a wide, shallow, radiating depression ; epidermis 

 greenish-yellow to brownish, feebly rayed; pseudocardinals 

 small, ragged, generally double in each valve ; laterals short, 

 more or less double in both valves : muscle scars impressed ; 

 nacre white or straw-colored. Male shell varying from subtri- 



