QUADRULA SII 
Genus OQUADRULA (Rafinesque, 1820) Agassiz. 
~ 
Quadrula RAFINESQUE, Ann. Gen. Sci. Phys. Brux., 1820, p. 
305.—OrTMANN, Ann. Car. Mus., VIII, 1912, p. 250. 
Rotundaria Acassiz, Arch. fur Naturg, 1852, p. 48. 
Orthonymus AGassiz, Arch. fur Naturg, 1852, p. 48. 
Shell triangular, quadrate or rhomboid, solid, inflated, with 
rather prominent beaks, which are generally sculptured with 
a few coarse, irregular, subparallel ridges that are inflated 
where they cross the posterior ridge; posterior ridge ordinarily 
well developed; base often incurved in old specimens; disks 
sculptured or smooth; epidermis usually dull colored, dark 
and rayless, or feebly raved; hinge plate heavy, wide, flattened ; 
pseudocardinals solid, direct, ragged; laterals double in the left 
and single in the right valve, often with a small secondary 
lateral below the large one in the right valve; cavity of the 
beaks deep and compressed; dorsal scars under the hinge plate ; 
male and female shells alike. 
Animal having the inner gills the larger, generally free 
from the abdominal sac the greater part or all of their length; 
marsupium occupying all four of the gills throughout, the 
whole smooth and pad-like. 
Type, Quadrula metanevra Rafinesque. 
The forms, which I have placed in the genus Ouadriula, 
seem to me to be perfectly well characterized by differences 
of the animal and the shell, so much so that I have placed them, 
together with a few Asiatic species, in a group of higher than 
generic value. In almost all cases the shells of Quadrula are 
short and solid, they are quite commonly, though not always, 
inflated, and with scarcely an exception the beak cavities are 
decp. ‘This last character separates them almost invariably 
from the species of Pleuwrobema, in which the beak cavities 
are uniformly shallow. 
With but few exceptions the epidermis is dark, rough and 
rayless. The exception occurs in the section Quadrula, in 
which the epidermis is sometimes painted and bright. Two 
anomalous forms occur in the United States, the Unio cylindri- 
