PILEUROBEMA 759 
Unio decisus Lea, Tr. Am. Phil. Soc., IV, 1831, p. 92, pl. x11," 
Aseeesee@bs., ll. 1634, op. «102, ph” xu,’ figs 23.——CoNraAD, 
Monog., I, 1835, p. 6, pl. m1, fig. 2—HAnNtery, Biv. Shells, 
1843, p. 187, pl. xxu, fig. 21—Kusrer, Conch. Cab. Unio, 
1852, p. 41, pl. vu, fig..3—Cuenu, Ill. Conch., 1858, pl. 
xvill, figs. 3, 3a, 3).—ReeEve, Conch. Icon., XVI, 1864, pl. 
XVip fe 7 1. 
Margarita (Unio) decisus Lea, Syn., 1836, p. 21; 1838, p. 18. 
Margaren (Unio) decisus Lea, Syn., 1852, p. 26; 1870, p. 40. 
Pleurobema decisa SIMPSON, Syn., 1900, p. 752. 
Unio scalenius Say, Am. Conch., VI, 1834. 
Unio anaticulus Lea, Pr. Ac. N. Sci. Phila., I, 1861, p. 40; 
1 Ac: N.-ScioPhila., V,-1862, ‘p. 92, pl. x11, fig. 240; Obs., 
VIII, 1862, p. 96, pl. xin, fig. 240.—Sowerpsy, Conch, Icon., 
OV 1866) pl. xxvii -fioy 190. 
Margaron (Unio) anaticulus Lea, Syn., 1870, p. 40. 
Unio consanguineus Lea, Pr. Ac. N. Sci. Phila., VI, 1861, p. 
boule, N-Scenremla WV; 1862)0p. 67) pl! vin, figs 217; 
Obs., VIII, 1862, p. 71, pl. vu, fig. 217——SowerrsBy, Conch. 
Icon., XVI, 1868, pl. Lxxvitt, fig. 409. 
Margaron (Unio) consanguineus Lea, Syn., 1870, p. 40. 
Unio crebrivittatus Lea, Pr. Ac. N. Sci. Phila., VI, 1861, p. 
Goris Ac. Ni. Ser Phila Vi 18606, p. 43, ‘pl. xv; fig. 41; 
Obs., XI, 1867, p. 47, pl. xv, fig. 41. 
Margaren (Unio) crebrivittatus Lea, Syn., 1870, p. 40. 
?Unic medius Reeve, Conch. Icon., XVI, 1864, pl. xvu, fig. 77 
I have united a number of nominal species under the oldest 
name. There is considerable variation in the form, the anterior 
end being often rounded in young or almost adult shells, while 
in old ones it is obliquely and strongly cut away below, leav- 
ing the beaks to project well in advance of the rest of the 
shell. The posterior end is generally bluntly pointed, but 1s 
sometimes widely rounded, young shells or those almost adult 
often have broad, dark green rest marks; they are rarely rayed, 
but often have one or two irregular blotched rays on the hinder 
part of each valve. 
