774 PLEWROBEMA 
pseudocardinals small, triangular, radial, roughened; laterals 
short, heavy ; nacre white, brilliantly iridescent posteriorly. 
Animal apparently having ovisacs in pairs, which are slight- 
ly wavy and lirate at the base; gills rather large, semicircular, 
inner much the larger, partly free from the abdominal sac; 
mantle thin, thickened at edge, and bordered with a dark line. 
PLEUROBEMA SHOWALTER (Lea). 
Shell subtriangular to suborbicular, inflated, very solid, in- 
equilateral, or equilateral; beaks high and full; posterior ridge 
generally well developed, narrowly rounded, usually ending 
in a blunt point below the median line; anterior end rounded 
or slightly subtruncate above, base more or less rounded ; pos- 
terior outline curved ; surface with irregular growth lines ; epi- 
dermis tawny to dark greenish-brown, sometimes feebly rayed ; 
pseudocardinals subtriangular, two in the left valve and one 
to three in the right; left valve with two short laterals; right 
valve with a double one; beak cavities very shallow; muscle 
scars smal!, deep; nacre whitish, sometimes tinted with purple, 
thicker in front. 
Length 32, height 30, diam, 20 mm. 
Length 31, height 33, diam. 24 mm. 
Coosa River, Alabama. 
Type locality, Coosa River, Wetumpka, Ala. 
Unio showalterii Lea, Pr. Ac. N. Sci. Phila., [V, 1860, p. 307 ; 
jiAc NeSer, Phila.“ 1862; *p:-73,-pl) vin, te 223. Obs. 
VIII, 1862, p. 77, pl. vin, fig. 223.—Sowersy, Conch. Icon., 
XVI, 1868, pl. 1.xxx1, fig. 426. 
Margaron (Unio) showalterti LEA, Syn., 1870, p. 55. 
Pleurobema showalterii SIMPSON, Syn., 1900, Pp. 757. 
This species varies from subtriangular to almost orbicular 
and considerably in the degree of inflation. It is generally 
dark greenish-brown, though occasional specimens show a 
flush of tawny-color. Some shells appear almost exactly on a 
casual view like young Quadrula solida or pyramudata, but the 
almost total want of beak cavities will at once distinguish them 
and the shell is solider than the voung of either of them. 
