QUADRULA 903 
Subgenus RoruNDARIA (Rafinesque, 1820) Simpson. 
Rotundaria RAFINESQUE, Am. Gen. Sci. Phys. Brux., V, 1820, 
p. 308.—OrtTMANN, Ann. Car. Mus., VIII, 1912, p. 257. 
Shell rounded or quadrate, slightly truncate above in front ; 
posterior ridge low; beaks prominent, curved inward and for- 
ward over a strongly marked lunule; beak sculpture consist- 
ing of numerous (20 to 30) fine, irregular, broken, somewhat 
concentric corrugations, which extend well on to the second 
year's growth and gradually blend with the regular sculpture ; 
posterior three-fifths of the shell tuberculate ; epidermis brown ; 
beak cavities very deep, compressed ; nacre violet. 
Animal having the gills very large, inner very much wider 
in front, free the whole length from the abdominal sac; bran- 
chial opening immense, with many small papillz; anal open- 
ing very large, smooth; superanal opening not at all closed 
below. 
Type, Obliquaria tuberculata Rafinesque. 
Ortmann, (1. c.), raises this group to generic rank. 
Group of Quadrula tuberculata. 
Shell rounded, truncated behind, with a wide depression 
down the post slope. 
QUADRULA TUBERCULATA (Rafinesque). 
Shell subelliptical to subquadrate, subcompressed or only 
slightly inflated, inequilateral, solid or ponderous; beaks ele- 
vated but not inflated, turned forward, rather sharp, their 
sculpture consisting of numerous fine, corrugations that are 
sometimes broken and doubly looped or zigzagged; anterior 
end usually evenly rounded; base. line curved; posterior end 
squarely or obliquely truncated with a well-marked sinus above 
the posterior ridge, usually angled behind the ligament; pos- 
terior ridge low, above it is a wide radial depression that ends 
in a notch or sinus; posterior three-fifths of the shell more or 
less tuberculate; anterior third concentrically sculptured; epi- 
dermis reddish-brown or greenish-brown, scarcely shining; 
