Call — The Unionidoe of Arkansas. 7 



incrassate; umbones slightly elevated, so much eroded that 

 minute characters are indeterminate ; ligament large, thick, 

 black, or dark brown ; epidermis yellowish horn-color, smooth, 

 polished, rayed with dark green over the whole disk, the rays 

 often interrupted by the lines of growth, which are numerous, 

 but somewhat indistinct; umbonal slope rounded, depressed 

 in the male, slightly elevated in the female ; posterior outline 

 emarginate in the female ventrad of the siphonal area, dorsal 

 outline rounded ; cardinal teeth double in the left and single 

 in the right valve, short, erect, triangular, solid, smooth, or 

 scarcely crenulate; plate connecting laterals with cardinal 

 teeth thick, somewhat arched ; lateral teeth rather short, thick, 

 slightly curved, smooth; anterior cicatrices distinct, large, 

 deeply impressed ; posterior cicatrices confluent, well im- 

 pressed, that of the retractor pedis muscle at tip of base of 

 lateral tooth but not on it ; dorsal cicatrices numerous and 

 deeply impressed in the cavity of the umbones ; nacre salmon 

 colored, occasionally white. Length 71.00 mm. ; breadth 

 27.20 mm.; height 45.50 mm. 



Animal dirty, yellowish white; labial palps short, ovately 

 triangular, adherent at base, laterally united so as to form an 

 oval groove, midway from the extremities of which is placed 

 the mouth. In the specimens examined only the anterior one- 

 third of the external branchiag contained ova. This portion 

 was characterized by the heavy deposit of pigmentary matter 

 at the apex of the chambers, while the remaining margins of 

 the branchiae were uniform in coloration with the mass of the 

 animal. The posterior borders of the mantle were, as usual, 

 differentiated into a series of tentacular folds ; those surround- 

 ing the incurrent and excurrent orifices were yellow and 

 brown, the remainder were black. 



While the females sustain a general resemblance to Ujiio 

 darhianus Lea and Unio gerhardtii Lea the emarginate char- 

 acter of the female form is utterly unlike anything exhibited 

 by the females of Lea's types. 



The above description is repeated from the original, that this 

 form, which has recently been found abundantly near Clinton, 

 Arkansas, may not remain unknown to those persons in that 

 State who take any interest in its natural objects. 



