820 QUADRULA 
terior ridge low and rounded; anterior end rounded ; basal and 
dorsal lines curved; posterior end obliquely truncated above, 
pointed or rounded below; surface concentrically sculptured 
and having uneven, oblique folds on the hinder half of the 
disk, which run nearly parallel with the posterior ridge and 
are sometimes broken into corrugations or pustules; dorsal 
slope usually having radial plications; epidermis greenish or 
yellow-green and sometimes rayed in young shells, becoming 
brown or blackish with age; pseudocardinals strong and rag- 
ged, two in the left valve and two or three in the right; left 
valve with two laterals; right valve with one; beak cavities 
deep, compressed; muscle scars shallow, the anterior partly 
filled with rough nacre; pallial line deep, remote; nacre whit- 
ish, iridescent and thinner behind. 
Length 165, height 108, diam. 60 mm. 
Length 140, height 97, diam. 60 mm. 
Length 136, height 93, diam. 42 mm. 
Mississippi basin generally; St. Lawrence drainage; Red 
River of the North; Lake Winnipeg; Alabama River system. 
Type locality, Ohio River. 
Unio undulatus Barnes, Am. Jl. Sci., VI, 1823, p. 120, pl. 11— 
Hantry, Biv. Shells, 1843, p: 175, pl. xx, fig. 26—CHENU, 
ib; Conch, rst -ser:, TM; 1845, p: 24,°pl. v, figs! 1, 10; Man- 
ual, 1859. Il, p. 143, fig. 704.—Sowerrsy, Conch. Icon., XVI, 
1868, pl. LXXVvI, fig. 390. 
Margarita (Unio) undulatus Lia, Syn., 1836, p. 12; 1838, p. 
T4. 
Margaron (Unio) undulatus Lea, Syn., 1852, p. 20; 1870, p. 
20. 
Mya undulata Eaton, Zool. Text-Book, 1826, p. 219. 
Unio undulata VALENCIENNES, Rec. Obs. Zool. Anat., II, 1833, 
p. 229, pl. Liv, figs. 3, 3a, 3b.—Desuayves, Tr. Elem. Conch., 
1920; Pp. tOsple xxx, f1es.8, 10: 
Ouadrula undulata BAKEr, Moll. Chicago, Pt. I, 1898, p. 82,. 
pl. xxu1, figs. 1, 2; xm, fig. 1—Srmpson, Syn., 1900, p. 769. 
Crenodonta undulata Orrmann, Ann. Car. Mus., VIII, 1912, 
p. 246. 
