UNIO 645 
ing behind below the median line in a somewhat rounded bian- 
gulation; dorsal-slope obliquely truncated; basal line usually 
incurved ; surface smoothish, or marked with irregular growth 
lines: epidermis greenish-yellow to tawny-brown, red-brown 
or black, feebly rayed, smooth and shining in young or finely 
preserved adult specimens, rough and dark in old ones; left 
valve with two low, small but stumpy, pseudocardinals and two 
remote laterals; right valve with one pseudocardinal and one 
lateral; beak cavities very shallow; muscle scars not deep; 
nacre purple, rarely white, iridescent behind, usually dull in 
front. 
Length 63, height 29, diam, 14 mm. 
Length 50, height 23, diam. 13 mm. 
Florida; South Georgia; west to the Black Warrior River, 
Alabama. 
Type locality, Black Warrior and Alabama Rivers. 
Unio arctatus Conrad, Am. Jl. Sci., XXV, 1834, p. 340, pl. 1, 
fig. 9; New F. W. Shells, 1834, p. 36, pl. v, fig. 4—HANLEy, 
Biv. Shells, 1843, p. 207, pl. xxi, fig. 47,—CHENU, Bib. 
Conch., Ist ser., III, 1845, p. 19, pl. 111, fig. 10.—KuSTER, 
Conch. Cab. Unio, 1861, p. 195, pl. Lx, fig. 3.—Srmpson, 
Syn., 1900, p. 717. 
Margarita (Umo) arctatus LEA, Syn., 1836, p. 38; 1838, p. 
Margarita (Unio) arctatus Lia, Syn., 1836, p. 38; 1838, p. 
This species has a very wide distribution and is exceeding 
abundant. Conrad’s shell is higher behind than in front, is 
compressed, black and quite elongated, as well as arcuate. 
There is every possible variation to shorter, lighter colored, 
more inflated forms, which are not at all arcuate. The species 
seems close to U. lazarus and, if it did not insensibly run into 
elliptical or obovate forms in Florida, I should place it in the 
group of U. complanatus. 
iS) 
to 
Sie 
5. 
ly 
Var. tortivus Lea. 
Under this name it may perhaps be well to place the ordi- 
nary southern manifestation of this protean species, which 
generally is not quite so elongated or arcuate as the type. In 
