CaU — The Unionidoe of Arkansas. 49 



tion of the Baron de Ferussac, surrendered his species to 

 Lamarck. 



From the examples in our cabinet, formerly belonging to 

 Troost, the following description is made: — 



Shell subcircuhir in outline, globose, convex, the longitudi- 

 nal about twice the lateral diameter, the male somewhat com- 

 pressed, rounded before, and angular behind, the female 

 somewhat emarginate posteriorly; epidermis thin, olivaceous, 

 greener over the umbones, eradiate, striate towards the mar- 

 gin, velvety; lines of growth numerous, and crowded, especially 

 so near the region of the margin, often broad and darker col- 

 ored ; posterior-umbonal slope separated from the lateral slope 

 by a well-marked angulation, with two rather faint carinas ; 

 umbones small, scarcely rounded, depressed, without undula- 

 tions; ligament short, rather thin, scarious, light brown, 

 curved with the dorsal margin; cardinal teeth short, thick, 

 erect, slightly inclined forwards, double in the left, and dis- 

 posed to be]trifid in the right valve, the central mass of the right 

 tooth far the largest ; the plate connecting the cardinals with 

 the laterals is poorly developed, thin, rounded, smooth ; lateral 

 teeth short, distant from the cardinals, straight, single in the 

 right and double in the left valve, crenulate, rather thin; na- 

 terior cicatrices distinct, deep, and pit-like, roughened, some- 

 what excavated under the plate formed by the cardinal teeth ; 

 pallial cicatrix faintly impressed throughout ; posterior cicatri- 

 ces very large, confluent, that of the retractor pedis at extreme 

 tip of lateral teeth; dorsal cicatrices small, arranged in a more 

 or less regularly disposed row within the cavity of the beaks, 

 pit-like and deep;^nacre purplish or rose-tinted, — this colora- 

 tion is most marked on the teeth and extends as a mere blush 

 of pink or rose over the balance of the shell. One specimen, 

 from Shreveport, is pure white except a portion of the lateral 

 teeth and the tips of the cardinals, approaching most nearly 

 Lamarck's description of the silvery nacre. Dimensions of ma- 

 ture male: length, 61.60 mm.; breadth, 31.32 mm. ; height, 

 45.58 mm. ; of mature female : length, 54.20 mm. ; breadth, 

 32.70 mm. ; height, 41.02 mm. 



This species will doubtless yet be found in southeastern 

 Arkansas, in muddy bayous. Its close resemblance to a 



