QUADRULA 879 ; 
Unio riddellii Lea, Pr. Ac. N. Sci. Phila., V, 1861, p. 392; Jl. 
ac, N; ect. Phila. V,. 1862;p. 198, pl..xxvit, fig. 267: Obs., 
1X, 1863, p. 20, pl. xxvu, fig. 267—Sowrrzy, Conch. Icon., 
XVI, 1868, pl. LXxxi1I, fig. 442. 
Margaren (Unio) riddell Lua, Syn., 1870, p. 38. 
Quadrula riddellii SIMPSON, Syn., 1900, p. 787. 
Pleurobema riddellii ORrTMANN, Ann. Car. Mus., VIII, 1912, 
Pp 262. 
This seems to be a rare species and I have never séen more 
than four or five specimens that I could refer to it with any 
certainty. The overhanging of the posterior truncation and 
the rounded base will distinguish it from Q. friersoni. It is 
more inflated in the middle of the disk than that species or 
chunu. Yet some intermediate material that I have seen 
hints at a connection with the latter. 
QUADRULA._ FRIERSONT (B. H. Wright). 
Shell triangular, inflated, solid, nearly or quite equilateral ; 
beaks very high and full, their sculpture apparently consisting 
of numerous oblique corrugations; posterior ridge high, an- 
gled, ending near the base of the shell; anterior end obliquely 
subtruncated above. rounded below; base line in adult shells 
nearly straight; outline of dorsal slope curved, often elevated 
into a low angle behind the ligament; surface irregularly, con- 
centrically striate; epidermis varying from ashy-brown to red- 
dish-brown; pseudocardinals triangular rough; lateral of 
right valve partly double; beak cavities moderately deep, com- 
pressed; muscle scars small, impressed; nacre whitish to pur- 
ple. 
Length 56, height 49, diam. 35 mm. 
Length 43, height 37, diam. 27 mm. 
Louisiana: eastern Texas. 
Type locality, Bayou Pierre, De Sota Parish, La. 
Unio friersoni B. H. Wricut, Nautilus, IX, 1896, p. 134, pl. 
IIT. 
Ouadrula-friersont Stmpson, Syn., 1900, p. 787. 
