338 KANSAS UNIVERSITY SCIBNCE BULLETIN. 



criminate between the variety and species in his list, it is im- 

 possible to make out the respective localities of his material. 

 There is hardly a Unio more subject to variation than this 

 one, particularly in the character of the epidermis and the 

 posterior outline. However, the relation of the height to the 

 length (approximately 1 to 2) and the dorsal posterior an- 

 gle remain remarkably constant. The species can best be 

 identified by the peculiarly undulated beaks and the broken 

 concentric bands of color of the epidermis. 



Genus PLEUROBEMA (Rafinesque, 1820) Agassiz. 



'* Shell solid, triangular to rhomboid, usually with a promi- 

 nent umboidal region ; beaks at or near the anterior end of 

 the shell, incurved and pointed over a small but well-developed 

 lunule ; beak sculpture coarse, consisting of a few irregular, 

 often-broken ridges with a curve upward posteriorly ; poste- 

 rior ridge present, but low and rounded ; epidermis showing 

 rest periods plainly, tawny to olive, often ornamented with 

 rays which show a tendency to break into square spots ; hinge 

 rather strong, the plate generally narrow ; pseudocardinals 

 double in both valves, in the right valve the inner being 

 smaller; muscle scars deep, the posterior rounded, cavity of 

 the beaks shallow ; nacre silvery ; male and female shells es- 

 sentially alike. 



"Animal having the inner gills much the larger, rounded 

 below, free from the abdominal sac for a part or all of their 

 length ; marsupium occupying the entire outer gills, the ovi- 

 sacs in some cases seeming to be arranged in pairs. Animal 

 generally yellowish to salmon-red, sometimes more or less 

 brown or blackish." (Simpson.) 



Pleurobema aesopus Green. Plate LXXVIII, fig. 2. 



Unio sesopus Green, Cont. Mac. Lyceum, i, No. 2, 1827, p. 46, fig. 3. 

 Unio cyphia Conrad, New Fresh-water Shells, 1834, p. 68. 



Shell large, thick, particularly anteriorly, roundly trape- 

 zoidal, tapering behind, rather inflated dorso-anteriorly. An- 

 terior margin fully rounded ; ventral margin gently and equally 

 curved, slightly emarginate posteriorly; posterior margin 

 roundly biangulate ; dorsal margin straight and somewhat 



