QUADRULA 875 
A peculiarly quadrate form, the posterior end being trun- 
cated and slightly rounded, the upper portion overhanging. 
The posterior ridge is curved and sharp throughout and the 
surface is rough. It seems to be rather rare. I have seen spec- 
imens larger than the one whose measurements are given 
above. It is more quadrate and less inflated than chunit. 
QUADRULA RUBIDA (Lea). 
Shell long rhomboid, convex to inflated, rather solid, inequi- 
lateral ; beaks high, more or less inflated, their sculpture con- 
sisting of numerous corrugations that curve up behind; pos- 
terior ridge full, sharp above, narrowly biangulate below, end- 
ing at the shell’s base in a point or double angle; anterior end 
rounded, sometimes obliquely truncate above ; base line straight 
or incurved in front of the posterior ridge; outline of dorsal 
slope curved or raised to a decided angle behind the ligament ; 
surface covered with strong, concentric ridges; epidermis 
greenish to reddish-brown, sometimes feebly rayed in the 
young shell; pseudecardinals triangular, rough; lateral of right 
valve single or partly double; beak cavities moderately deep, 
compressed ; muscle scars impressed ; nacre white or flesh-color. 
Length 82, height 60, diam. 40 mm. 
Length 73, height 55, diam. 34 mm. 
Length 70, height 48, diam. 24 mm. 
Streams flowing into the Gulf of Mexico from Alabama to 
Louisiana. 
Type locality, Tombigbee River, Miss.; Coosa River and 
Big Prairie Creek, Ala. 
Unio rubidus Lea, Pr. Ac. N. Sci. Phila., V, 1861, p. 40; Jl. 
Ace (Nee Sciaohilap se tsG2rn ps OSaspl. \ xiv, fies. 244-5 Obs., 
VIII, 1862, p. 99, pl. xiv, fig. 244. 
Margaren (Unio) rubidus LEA, Syn., 1870, p. 35. 
Ouadrula rubida SIMPSON, Syn., 1900, p. 787. 
Unio negatus Lea, Pr. Ac. N. Sci. Phila., V, 1861, p. 59; JI. 
nee, sci, Phila. VW; 1662. pe Zo, pl. 1x, fig.225; Obs., VILL, 
1862, p. 80, pl. 1x, fig. 225.—Sowersy, Conch. Icon., XVI, 
1866, pl. Xxx, fig. 165. 
Margaron (Unio) negatus Lua, Syn., 1870, p. 35. 
