UNIO 599 
Var. arcus Conrad. 
Shell small, thick and ponderous, subintlated, arcuate. 
Length 50, height 25, diam. 21 mm. 
Alabama River drainage, probably merges into subgibbosus. 
Type locality, Alabama River. 
Umio arcus Conran, Am. Jl. Sci. XXV, 1834, p. 340, pl. 1, fig. 
8.— Han ey, Biv. "Shells, 1843, p. 267, pl. xxi, fig. 46. 
Margarita (Unio) arcus LEA, Syn., 1836, p. 38; 1838, p. 25. 
Margaron (Umo) arcus LEA, Syn., 1852, p. 38; 1870, p. 61. 
Unio gibbosus var. arcus SIMPSON, Syn., 1900, p. 704. 
Var. subgibbosus Lea. 
Shell small, solid, inflated or subinflated, with a high pos- 
terior ridge. Typically subrhomboid and slightly full on the 
base line. 
Length 54, height 29, diam. 18 mm. 
Alabama River system; Saline River, Arkansas. 
Type locality, Oostanaula, Floyd Co. ; Etowah River, Ga. 
Unio subgibbosus Lea, Pr. Ac. N. Sci. Phila., IX, 1857, p. 
160 4, [Aga N — Sei: Philas, WV ,, 1858;. p:-53, «pl. vi, fig: 26; 
Obs., VI, 1858, p. 53, pl. v1, fig. 36—Srmpson, Pr. U. S. 
Nat. Mus., XV, 1892, p. 432, pl. Lxxvitt, fig. 5. 
Margaron (Unio) subgibbosus LEA, Syn., 1870, p. Ot. 
Unio lasarus Sowerby, Conch. Icon., XVI, 1868, pl. Lxvut, 
fig. 348. 
Umiovrnjus LRA Pre AGN. Sch. hild.,, 1X,’ 1857, p.171.;. JI. 
Aer N-voct. Phila TV, 1858;:p.-85;.pl: xvi, fic. 6s. 
Margaron (Unio) rufus LEA, Syn., 1870, p. 61. 
Unio gibbosus var. subgibbosus Stmpson, Syn., 1900, p. 704. 
Varies from the above form into arcus and armathwaitensis. 
Var. armathwaitensis B. H. Wright. 
Shell narrowed in front, widest behind, subcompressed, sub- 
solid. 
length 87, height 40, diam. 19 mm. 
length 65, height 34, diam. 17 mm. 
