PI.EUROVEMA 763 
base rounded in front, cbliquely truncate behind; posterior 
ridge low, ending behind in a point about on the median line ; 
in front of the center of the shell there is a wide, radial swell- 
ing; epidermis dark greenish-brown, greenish in the young 
shell, often showing faint rest marks; pseudocardinals trian- 
gular, radial, three in the right valve and two in the left; lat- 
erals two in the left valve and a double one in the right ; beak 
cavities shallow ; muscle scars small, impressed; nacre bluish- 
white, thin and iridescent behind. 
Length 47, height 34, diam. 24 mm. 
Type locality, Tombighee River, Columbus, Mississippi. 
Uniaveurins Ins Pr. Ac. IN. Sei Phila., Tl; 1850,. p..112); Jl. 
New Noein bila: Vi51S02, pxlO3.pl.. XVvil, fig. -253;< Obs., 
VEE S625 p.4107,.pl. xvii fie: 252: 
Margaron (Unio) curtus Lea, Syn., 1870, p. 40. 
Pleurobema curta SIMPSON, Syn., 1900, p. 754. 
The radial swelling at and in front of the middle of the shell 
is strong and wide. Behind it the shell is compressed and the 
base line is straight or even a little incurved. The anterior 
truncation is square. ‘The form and the dark epidermis will 
distinguish it from all other species. 
Group of Pleurobema troschelianum. 
Shell rather small, solid, rounded-triangular, inflated, some- 
what inequilateral, slightly pointed near the posterior base, 
and rounded angular at the hinder end of the ligament; base 
of the shell almost evenly rounded; posterior ridge low; beaks 
high and slightly curved inward and forward over a well-de- 
fined lunule; epidermis dull tawny, showing the rest periods 
and occasionally marked with a radiating row of dark green, 
squarish spots in front of the posterior ridge; hinge plate wide 
and ‘lat; pseudocardinals small, triangular, rough; laterals 
short. curved: muscle scars small, rather deep. Animal with 
semicircular gills, the inner the larger and free from the ab- 
dominal sac half to nearly their whole length; ovisacs of outer 
gills apparently in pairs. 
