Call — The Unionidce of Arkansas. 13 



Habite VAmerique septentrionale ^ dans le Mississippi, 

 V Ohio y et plusieurs lacs. * * *" 



The remainder of the description is concerned with the 

 diagnoses of the varieties which Lamarck considered as belong- 

 ing to this form. Variety a is from the Mississippi; variety 

 h from Lake Erie, variety c from the Ohio. But each variety 

 appears to be a distinct species. 



The following description is based upon specimens obtained 

 from the Cumberland river, at Nashville, Tennessee, where the 

 species is very abundant; also facts are included from char- 

 acters exhibited by abundant material from the Etowah and 

 Oostanaula rivers, in Georgia. The species is likewise abun- 

 dant in the Cahaba, Alabama, and Coosa rivers, in Alabama. 



Shell smooth, elliptical, compressed, incrassate anteriorly, 

 biangular and much thinner posteriorly ; epidermis rather 

 thick, black in old specimens and deep reddish brown in 

 young ones, striate, often with curved, dark green rays ex- 

 tending ventrad from the umbones, in the young shell; the 

 dorso-posterior margin much and quite regularly-curved ; 

 posterior umbonal slope eradiate, somewhat flattened, sep- 

 arated from the lateral slope by a marked angle, with a prom- 

 inent raised line, sometimes two, extending from the umbones 

 and joining the posterior margin at the angles, the whole pos- 

 terior slope is, commonly, strongly and coarsely striate ; um- 

 bones small, scarcely prominent, slightly incurved; ligament 

 long, thick, curved with dorsal margin, black ; cardinal teeth 

 short, heavy, triangular, striate, single in the right, double 

 in the left valve, the posterior portion of the double left 

 tooth nearly equal in size and shape to the single right tooth ; 

 lateral teeth long, thick, straight or nearly so, crenulate, in 

 old specimens this is strongly marked ; dorsal plate connect- 

 ing the lateral with the cardinal teeth scarcely marked, 

 smooth, rounded ; anterior cicatrices distinct, deeply im- 

 pressed, that of the adductor muscle much roughened and 

 pitted with numerous small pits arranged in a row near the 

 edge of the plate forming its upper margin; pallial cicatrix 

 well impressed anteriorly and markedly crenulate throughout ; 

 posterior cicatrices distinct, that of the adductor deeply im- 

 pressed and extending to the posterior end of lateral teeth. 



