750 PILEUROBEMA 
green rays that break up into squarish blotches, scarcely shin- 
ing ; pseudocardinals small, stumpy, two in the left valve, three 
in the right; laterals two in the left valve and a somewhat 
double one in the right; beak cavities shallow; muscle scars 
small, deep; nacre bluish-white, thin and iridescent behind. 
Length 28, height 21, diam. 18 mm. 
Type locality, Nolachucky River, ‘l’ennessee. 
Umno tesserule les, Pr Aci Ne Scr Phila, Vie 186i, page 
Ji, Ac: IN. Set. Phila., VI, 1866, "p: 40,-pl: xy, fig: 39+) Obs: 
XI, 186075 p. 44, pl xvotie) 30. 
Margaron (Unio) tesserule Lea, Syn., 1870, p. 36. 
Pleurobema tesserule Simpson, Syn., 1900, p. 749. 
A well-characterized little species, which seems to be quite 
rare, as | have only seen two or three authentic specimens. It 
is much inflated, is decidedly rhomboid; it gapes rather widely 
at the anterior base and the hinder part of the nacre is thin, 
and iridescent, almost translucent and has a purplish tint. It 
seems to be a healthy, normal species. 
PLEUROBEMA VALIDUM (Lea). 
Shell subtriangular, scarcely inflated, solid, inequilateral ; 
beaks high, rather full; lunule narrow; anterior end very 
slightly, obliquely truncated above, widely rounded below 
base line curved; outline of the dorsal slope curved, raised al- 
most to an angle at the hinder part of the ligament; posterior 
ridge narrowly rounded, straight, ending below in a blunt 
point near the base of the shell; surface with uneven, concen- 
tric sculpture ; epidermis tawny-greenish, with numerous, rath- 
er faint, scarcely broken rays; pseudocardinals rough, radial, 
two in the left valve and three in the right, the middle 
one in the right valve strong; left valve with two laterals; 
right valve with a somewhat double one; beak cavities shallow ; 
muscle scars small, deep; nacre silvery, slightly iridescent be- 
hind, thicker in front. 
Length 55, height 45, diam. 26 mm. 
‘Tennessee. 
Type locality, Duck River, Tenn. 
