PILEUROBEMA 733. 
lunule: beak sculpture coarse, consisting of a few irregular, 
often broken ridges, which curve upward posteriorly; pos- 
terior ridge present, but low and rounded; epidermis showing 
the 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 tri- 
angular, ragged: laterals reaching nearly or quite to the pseu- 
docardinals, double in both valves, in the right valve the inner 
being smaller; muscle scars deep, the posterior rounded; cav- 
ity of the beaks shallow; nacre silvery ; male and female shells 
essentially alike. 
Animal having the inner gills much the larger, rounded be- 
low, 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. 
Type, Unio clava Lamarck. 
The species which I have placed under the generic name 
Pleurobema seem to stand between Quadrula and Unio. ‘The 
heavy inflated forms of the clava, showalterti and troscheliana 
groups approach Quadrula in some cases so closely that it is 
difficult to separate them from it. But all have shallow beak 
cavities, while those of Ouadrula are almost invariably deep, 
and I have no doubt that all carry the young in the outer gills 
alone, instead of in all four as the Quadrulas do. On the 
other hand many species of the argentea group and a few in 
the clava and other groups approach Unio so closely that I 
have been at a loss to know where to place them. Most of 
the shells of the genus, though not all, have a tawny-colored 
epidermis often marked with broken, bright green rays that is 
different from the species of either Unio or Quadrula and 
nearly all of them are confined to the Ohio, Tennessee and 
Alabama River systems. ‘Two or three forms whose relations 
are a little doubtful belong in the Atlantic drainage. 
