PLEUROBEMA 769 
has several older shells with the same name, but most of them 
are perhaps something else. The species is more evenly ellip- 
tical than P. irrasum, but I am doubtful whether it is distinct. 
PIEUROBEMA STABILE (Lea). 
Shell subovate, subelliptical or subtriangular, solid, inflated, 
somewhat inequilateral; beaks high, full and rounded; lunule 
narrow ; anterior end evenly rounded; base line curved; out- 
line of the dorsal slope curved, sometimes raised behind the 
ligament into a low angle; posterior ridge rather low, slightly 
double below, ending at and below the median line in a faint 
biangulation; surface with irregular, concentric striz ; epider- 
mis tawny or tawny-brownish, rarely with a broken row of 
squarish green blotches: pseudocardinals ragged, triangular, 
two in the left valve and one to three in the right; laterals 
two in the left valve and a double one in the right; beak cav- 
ities shallow; muscle scars small, impressed; nacre whitish, 
slightly iridescent behind. 
Length 36, height 27, diam. 22 mm. 
Length 43, height 33, diam. 27 mm. 
Type locality, Coosa River, Alabama. 
Umino staihsyiean, Pr: Ac. .N. sei. Phila., V,. 1861, p. 59; Jl. 
ie Ne oci Phta.2V  Eso2./pie71, pk. viii, fis) 221; Obs. 
VIII, 1862, p. 75, pl. vii, fig. 221. 
Margaron (Unio) stabilis Lea, Syn., 1870, p. 38. 
Pleurobema stabilis SIMPSON, Syn., 1900, p. 755. 
Umo medus Lua, Pr. Ac. N. Sci. Phila., V., 1861, p./ 40% Jl. 
ACN. oc Emila.), 1802, p. 7euplax, fig, 227:;-Obs.,. VEIL, 
1862, p. 82, pl. x, fig. 227.—Sowerpy, Conch. Icon., XVI, 
1500, pl Lxxmr, fic. 375: 
Margaron (Unio) medius Lea, Syn., 1870, p. 40. 
A species with no very striking characters. Lea’s Unio 
medius seems to me to be only the younger shell of his stabilis, 
which appears to be adult. The high, full, rounded beaks 
placed at some distance back from the anterior end sometimes 
give the shell a somewhat triangular outline. ‘The double pos- 
terior ridge and faint biangulation where it ends behind are 
constant characters in all the specimens IT have seen. 
