g16 QUADRULA 
be rhomboid: each valve with a posterior ridge, that on the 
valve pushed forward less developed; beaks high, full, appar- 
ently sculptured with a few coarse, irregular ridges; posterior 
slope usually having strong, radiating, upcurved ridges; hinge 
plate generally wide and flattened; pseudocardinals heavy, 
somewhat radiate, often with the sockets evenly, radially 
grooved; laterals strong, heavily vertically ridged; secondary 
lateral of right valve well developed; anterior cicatrices small, 
deep, posterior shallow ; cavity of the beaks enormously deep, 
compressed ; dorsal cicatrices on the under side of the shell. 
Animal unknown. 
QUADRULA TORTUOSA (Lea). 
Shell subelliptical, inequivalve, very inequilateral, twisted, 
inflated, solid; beaks prominent, terminal, swollen; anterior 
end cut away below; base line curved; outline of posterior end 
curved and cut away below; dorsal slope with a gently curved 
outline; left valve pushed either forward or back on the right; 
posterior ridge strongest on the valve projecting behind; dor- 
sal slope having subradial, curved, broken corrugations; sur- 
face with concentric growth lines but lacking nodules; epider- 
mis somewhat silky when fresh, ashy-brown; pseudocardinals 
large, heavy, elongated, running almost parallel with the hinge 
line, a heavy posterior and a smaller split one in the left valve 
with one in the right valve; lateral of right valve single or 
partly double, vertically striate; anterior muscle scars small, 
deep ; beak cavities very deep, compressed ; nacre silvery white. 
Length 67, height 47, diam. 35 mm. 
Length 72, height 46, diam. 37-mm. 
China. 
Unto tortuosus Lea, Pr. Ac. N. Sci. Phila., IX, 1865, p. 76; 
Jl. Ac. Ny Set Phila.; Vi, 1868; ps 286.) pl xxx, fis es 
Obs., XII, 1860, p. 46. pl. xxx1x, fig. 98. 
Margaron (Unio) tortuosus Les, Syn., 1870, p. 30. 
Ouadrula tortuosa SIMPSON, Syn., 1900, p. 798. 
Unio (Lampsilis) subtortus Barry and Apams, Proc. Zool. 
Soc. Lond., 1867, p. 491, pl. xxv1, figs. I, Ta. 
