﻿HEMTI,ASTENA 323 



front there is a wide, shallow, radial depression ; posterior end 

 truncated ; surface cov^ered with strong, irregular tubercles, 

 those on the posterior slope partially arranged in curved rows ; 

 just at and back of the posterior ridge there are faint indica- 

 tions of knobs ; epidermis brown ; left valve with two strong 

 pseudocardinals, the posterior one bifid and two strong lat- 

 erals ; right valve with three pseudocardinals. the middle one 

 much the largest, and one double lateral ; anterior scars rough ; 

 posterior scars oblong ; beak cavities moderately deep, com- 

 pressed ; pallial line deep and roughened ; nacre lurid white, 

 much thicker in front, with two faint, wide, radial depressions, 

 one at the post-ridge, the other in front of the middle. 



Length 93.75, height 68.75, diam. 50 mm. 



Type locality, Hiawassee River, Tennessee. 

 Unio conjuiians B. ?I. Wright, Naut., XIII, iSrx), p. Sg. 

 Tritogonia conjugans Simpson, Pr. Ac. N. Sci. Phila., 1900., 



p. 79, pi. IV, fig. I. — Syn., 1900, p. 609. 



I am not at all certain as to the systematic position of this 

 species. The posterior truncation is much dififerent from that 

 of T. tiibcrailata, but the epidermis and tubercles, the hinge, 

 beak cavities and pallial line are very much as in that species. 



A single male shell of this remarkable species, the type, is 

 in the U. S. National Museum collection. 



Genus HRMILASTENA (Agassiz, 1852) Simpson. 



Hemilastena Agassiz, Arch. fur. Nat., I, 1852, p. 50. — Simp- 

 son. Syn., 1900. p. 673. 



Shell small, elongate elliptical, rounded in front and behind, 

 often slightly incurved at the central base ; beaks rather sharp, 

 but not full ; sculpture consisting of fine, parallel ridges, which 

 are looped up in the middle and open behind ; epidermis brown- 

 ish, rayless ; teeth imperfect, a single irregular, compressed 

 tooth in each valve ; that of the left under the beak, that of the 

 right in front of it; laterals nearly or quite wanting; anterior 

 end of the shell much thickened ; anterior muscle scars united ; 

 posterior faint; nacre dull whitish. 



