646 UNIO 
many cases shells that I refer to this form are irregularly el- 
liptical or rhombic obovate. U. purpurellus seems to me to be 
only a small, rather inflated form of tortivus. 
Type locality, Chattahoochee River, Columbus, Ga. 
Unio. tortivus Leas Pry Amr PhilvSocal11840..p. 2875 
Am; Phil. Secs Villy r842apr2045-plasaiehicn1 7) @psss ile 
1842, p. 42, pl. xu, fig. 17—CueENu, Ill: Conch., 1858, pl. 
XXIX, figs. 6, 6a, 6b.—Sowerby, Conch. Icon., XVI, 1866, 
pl. xxxvi, fig. 193.—Simpson, Pr. U. S. Nat. Mus., XV, 
1892,-p422, pl. LAM, fige8; LX1V, hes. Ty 3304: 
Margaron (Umio) tortivus Lea, Syn., 1852, p. 33; 1870, p: 53- 
Unio arctatus var. tortivus SIMPSON, Syn., 1900, p. 718. 
Omo.tetncus Lea, Pr. Aco NE Sc Pina oe 1857. pate; 
jl Acs Ns Sci“ Phila, IVG1850;) po 1G5.ipl sem dies 7 Oerebse: 
VI. 1850,..p. 13,° pl. xxi, fig. 78. Kuster: 1Conche abt 
Unio, 18624 p:)276.pl? xer, ties 7.2 KCiliy ne 
Margaron (Unio) tetricus LEA, Syn., 1870, p. 53. 
Unio purpurellus Lea, Pr..Ac. N. Sci. Phila., 1X, 1857, .p. 171; 
Ji. Ac. N. Sei. Phila., TV, 1859, p. 19, pl. xx1n, fig: 81; Obs., . 
VIL) 18503, px 16, pla xxi, eS: 
Margaron (Unio) purpurellus Lea, Syn., 1870, p. 53. 
Unio merceri lasa, Pr. Ac. N. Sci. Phila., VI, 1862, p. 169; Jl. 
Ac: Ni: Ser. Pinlas VV, 1862,"p 200. pl. xxx, fre 275 Obs. 
DXE* 7863 pi3 tcp kx, hee e7G: 
Margaron (Unio) mercerit Lea, Syn., 1870, p. 61. 
UNIO NIGELLUS Lea. 
Shell subrhomboid, narrowed in front, subinflated, inequi- 
lateral ; beaks slightly elevated and inflated, their sculpture con- 
sisting of numerous corrugated, somewhat doubly-looped 
ridges ; posterior ridge full, double, ending behind in a feeble 
biangulation at or near the base of the shell. The shell is most 
strongly inflated along the posterior ridge, in front of which 
it is somewhat wedge-shaped; posterior slope obliquely trun- 
cate; base line nearly straight, sometimes a little full behind 
the middle; anterior end rounded and narrow; surface with 
irregular growth lines, dirty greenish and rayed in the young 
