798 PLEUROBEMA 
curved; outline of dorsal slope curved, elevated to a low angle 
behind the ligament: surface rudely, concentrically striate; 
epidermis concentrically wrinkled, reddish-brown, not shining ; 
left valve with two pseudocardinals and two laterals; right 
valve with two pseudocardinals and a double lateral; muscle 
scars impressed ; beak cavities not deep; nacre lurid flesh-color, 
shining, thinner behind. 
length 60, height 40, diam. 22 mm. 
Upper Tennessee River drainage. 
Type locality, Headwaters of the Holston River, Washing- 
ton Co., Va. 
Unio fassinans Lea, Pr. Ac. N. Sci. Phila., XII, 1868, p. 143; 
fl.-Acl ON. Ser Phila, Vil 860; p) 305s) pl.) sive. 
Obs) Xl p65, pl xvi, fee: 
Margaron (Unio) fassinans LEA, Syn., 1870, p. 42. 
Pleurobema fassinans SIMPSON, Syn., 1900, p. 762. 
Close to several forms of this puzzling group. It is more 
evenly elliptical than argentewm and has a darker epidermis. 
Var. rhomboideum Simpson. 
Shell considerably more rhomboid than the type. 
Type locality, Upper waters of the Clinch River, Virginia. 
Pleurobema fassinans var. rhomboidea SIMPSON, Syn., 1900, 
[Dn AOS 
A form occurs in this region, which has most of the char- 
acters of fassimans, but is more rhomboid. It may be a dis- 
tinct species but I do not feel like adding another specific 
name to this terribly difficult group without the very best of 
reasons. 
PLEUROBEMA ARGENTEUM (Lea). 
Shell subrhomboid or subovate, compressed or subcompress- 
ed, rather solid, inequilateral; beaks not very full or high; 
posterior ridge single or double, ending in a point or biangula- 
tion near the base line; anterior end rounded; base line 
curved; outline of dorsal slope raised to an angle behind the 
ligament, the posterior end being obliquely truncated ; surface 
with fine, uneven, concentric sculpture; epidermis dull green- 
