644 UNIO 
Unio occut'tus Lea. 
Shell somewhat obovate or subrhomboid, a little narrowed 
in front, inequilateral, subcompressed or convex; beaks only 
moderately full or high, their sculpture consisting of nearly 
longitudinal ridges; posterior ridges usually faintly double, 
ending at and below the median line in a biangulation; post- 
basal region generally a little produced ; surface nearly smooth, 
tawny-brown, usually closely covered with faint green rays, 
sometimes almost rayless and dark brown; left valve with two 
subcompressed pseudocardinals and two curved laterals; right 
valve with two pseudocardinals, the upper small and one lat- 
eral; muscle scars slightly impressed; nacre purple, sometimes. 
a little coppery-tinted. 
Length 52, height 28, diam. 18 mm. (type). 
Florida. 
Type locality, Black Creek and Lake Monroe, Fla. 
Unio occultus Lea, Desc. 12 Uniones, 1843 (no pagination ).— 
Tr. Am. Phil.-Soc.; 1X>.1845°;,\p. 270. plexus ne 7 Ops, 
IV, 1848, p. 37, pl. x11, fig. 7—Kustrer, Conch. Cab. Unio, 
1861, p. 223, pl. LXxv, fig. 3—Simeson, Pr. U. S. Nat. Mus., 
XV, 1892, p. 420, pl. Lx, figs. 5, 6; Syn., 1900, p. 717. 
Margaron (Unio) occultus Lima, Syn., 1852, p. 48; 1870, p. 48. 
An exceedingly puzzling form, which varies toward fuscatus, 
tortivus and even coruscus. It is generally a little solider, 
darker and more inflated than fuscatus and has a little differ- 
ent texture. It is not quite so rhomboid as most specimens of 
tortivus nor is it arcuate; it is less solid and inflated than corus- 
cus. But there seems to be a set of connecting links, which 
more or less perfectly unite the above and several other so- 
called species of this excessively variable and puzzling group. 
Unio arctatus Conrad. 
Shell elongated, compressed or subcompressed, usually ar- 
cuate, inequilateral. subsolid; beak low, compressed, their 
sculpture strong, corrugated ridges, which nearly follow the 
growth lines: posterior ridge low, usually faintly double, end- 
, 
