7 
PLEUROBEMA 773 
terior ridge is feebly developed and the disk in front of it is 
full. 
Dr. ea has shells in his collection from the Clinch River, 
which he places with this species, which I am certain belong 
to the clava group. . 
PLEUROBEMA INSTRUCTUM (Lea). 
Shell subtriangular, convex to subinflated, inequilateral, sub- 
solid: beaks high, rather full, their sculpture a few strong, ir- 
regular ridges that curve up decidedly behind; anterior end 
evenly rounded; base line slightly curved; outline of dorsal 
slope curved, raised into an angle behind the ligament; pos- 
terior ridge high, somewhat narrowly double, ending in a bian- 
gulation near the base: surface unevenly, concentrically striate, 
epidermis pale greenish-tawny ; pseudocardinals small, two in 
the left valve, one to three in the right ; laterals two in the left 
valve and one in the right. the latter somewhat double; beak 
cavities shallow ; muscle scars small, not deep; nacre white or 
bluish-white, silvery or iridescent behind. 
Length 38, height 30, diam. 17 mm. 
Length 35, height 28. diam. 19 mm. 
Type locality, Cahawba River, Alabama. 
Unaorinstructus Ea Pra Ac. IN. Sct. Phila. V, 1861, p. 59; 
iseAcw IN oct e aids. Vv .1S02. pp: o2.\pl. x, fie: 230% Obs., 
VI, 1862, p. 86, pl. x, fig. 230. 
Margaron (Unio) instructus Lea, Syn., 1870, p. 38. 
Pleurobema instructa SIMPSON, Syn., 1900, p. 756. 
Thinner and less inflated than any of the nearly related spe- 
cies. In fresh specimens the epidermis is often somewhat 
cloth-like. 
Group of Pleurobema showalterit. 
Shell small, rounded triangular, inflated, slightly truncated 
anteriorly, posterior end arched and ending in a bluntly round- 
ed point at the post-base, posterior ridge well defined ; umbonal 
region full; beaks rather prominent, the sculpture not observ- 
ed: epidermis brown, not rayed; hinge plate rather wide: 
