QUADRULA 881 
Unio trigonus Lea, Tr. Am. Phil. Soc., IV, 1831, p. 110, pl. 
XVI, fig. 40; Obs., I, 1834, p. 120, pl. xvi, fig. 40—HANLEy, 
Biv. Shells, 1843, p. 185, pl. xx1, fig. 6.—CueEnu, Ill. Conch., 
1858, pl. xvi, figs. 7, 7a, 7b.—Sowersy, Conch. Icon., XVI, 
1868, pl. LXXXVI, fig. 450. 
Margarita (Unio) trigonus Lira, Syn., 1836, p. 18; 1838, p. 17. 
Margaren (Umo) trigonus Lea, Syn., 1852, p. 25; 1870, p. 38. 
Quadrula trigona BaxeEr, Moll. Chicago, Pt. 1, 1808, p. 76, pl. 
Xv, fig. 5—S1mpson, Syn., 1900, p. 787. 
Unio triangularis KustER, Conch. Cab. Unio, 1852, p. 56, pl. 
EEL, LS. 
Unio pilaris Reeve, Conch. Icon.; XVI, 1865, pl. xxvu, fig. 138. 
Quadrula obliqua (part), Stmpson, Syn., 1900, p. 788. 
This abundant species approaches closely to several others 
but in all cases that I have seen there is a wide, shallow radial 
groove in front of the rather sharp, well-defined posterior 
ridge and in front of this depression the disk is swollen, so that 
along this swelling the shell has a much greater diameter than 
at the posterior ridge. There is usually a large, wide lunule, 
and often below this there is an illy-defined, flattened area, 
which almost forms a second lunule. In chunii and friersoni 
there is no such lunule-like depression and scarcely anything 
of the radial depression on the disk. Q. rubiginosa is gener- 
ally considerably less inflated than wndata and lacks the second 
lunule and radial depression, but there are intermediate shells 
that may be referred to one species as well as to the other. 
QUADRULA OBLIQUA (Lamarck). 
Shell subtriangular, inflated, solid, somewhat inequilateral ; 
beaks very high and full, turned inward and forward over a 
decided lunule, their sculpture a few, coarse, irregular ridges 
that are turned up behind, and nodulous on the posterior ridge; 
anterior end usually obliquely truncate above, rounded below; 
base line usually lightly sinused in front of the posterior ridge ; 
outline or dorsal slope generally almost evenly curved from the 
beaks to the base; posterior ridge rather low but well develop- 
ed, narrowly rounded, placed near the edge of the shell and 
curved throughout ; in front of the middle of the shell is a high. 
