882 QUADRULA 
wide, curved, radial swelling ; between this and the much lower 
posterior ridge is a wide, curved, radial depression; surface 
having irregular, concentric striz ; epidermis tawny-brown and 
raved in the young, dark reddish-brown in the old shells, dull; 
pseudocardinals ragged; lateral of right valve partly double; 
muscle scars and beak cavities deep; nacre white, thinner and 
somewhat iridescent behind. 
Length 115, height 95, diam. 57 mm. 
Length 95, height 78, diam. 50 mm. 
Length 77, height 69, diam. 40 mm. 
Ohio, Cumberland, and Tennessee River systems; west in 
Illinois to the Mississippi; Claiborne, Alabama. 
Type locality, Ohio River. 
Unio obliqua I.AMarcK, An. sans Vert., VI, 1819, p. 72. 
Margarita (Unie) obliqua Lea, Syn., 1836, p. 20. 
Quadrula obliqua Simpson, Syn., 1900, p. 788. 
Unio obliquus HANLEY, Biv. Shells, 1843, p. 186, pl. xx, fig. 
24.—KusTer, Conch. Cab. Unio, 1852, p. 63, pl. xv, fig. 1.— 
CALE, Ir. Xe. Ser St. Louise Ville r8o5 mare te eplcany: 
Margarita (Unio) obhiquus Lea, Syn., 1838, p. 17. 
Margaron (Unio) obliquus Lea, Syn., 1852, p. 25; 1870, p. 38. 
Pleurobema obliquum OrtTMANN, Ann. Car. Mus., VIII, 1912, 
Dp: 2045 
Unio mytiloides SHort and Eaton, Transylvania Jl., 1831, p. 
74- 
Unto cerdatus Conrap, Monog., V, 1836, p. 48, pl. xxv.— 
Kuster, Conch. Cab., 1852, p. 57, pl. x1, fig. 1.—SoweErsy, 
Conch: licon., SOV, 1868; ply L xxii, 376, 
The posterior ridge and outline of the dorsal slope are curv- 
ed. In wndata the posterior ridge is straight, and the post-dor- 
sal outline is raised to an angle in the middle. QO. obliqua ap- 
proaches more nearly to an equilateral triangle than either 
pyramidata or solida. It is less inflated and longer than 
plena. 
Ortmann, (1. c.), states that only the outer gills are marsupi- 
al, But Lefevre and ‘Curtis, (Bull, Bur Fish, XOGX, 1or2: 
p. 120), find that all four gills are used for that purpose. 
Ortmann also remarks that this species, in his opinion, is 
not specifically different from coccineum.” 
