760 P1LEUROBEMA 
PILEUROBEMA CHATTANOOGAENSE (Lea). 
Shell subelliptical, subinflated or inflated, solid, very inequi- 
lateral ; beaks full and high; lunule distinct ; posterior ridge low, 
often scarcely marked; anterior end feebly truncate or evenly 
rounded; base line curved; outline of dorsal slope generally 
more nearly straight than that of the base; posterior end 
rounded, sometimes bluntly pointed or slightly biangulate; 
outline of shell when viewed from above wedge-shaped; epi-: 
dermis tawny-brown, sometimes shaded green, usually having 
the rest-marks very distinct; these and one or two ill-defined 
rays on the posterior end are often dark green; pseudocar- 
dinals irregular, radial, two in the left valve and three in the 
right; laterals two in the left valve and a double one in the 
right ; beak cavities shallow ; nacre white, iridescent behind. 
Length 56, height 33, diam. 25 mm. 
Alabama River system. 
Type localitv, Chattanooga, Tenn.; Etowah, Oostanaula and 
Coosawattee Rivers, Ga. 
Unio chattanoogaensis Lmwa, Pr. Ac. N. Sci. Phila., I, 1858, p. 
166; Jl. Ac. NoSet. Phila. 20V;:1850;p. 200) pl. xv neetoer, 
Obs., VIT, 1859, p. 27, pl. xxv, fig. 90.—ReEEveE, Conch. Icon... 
XVI, 1864, pl. xvi, fig. 60. 
Margaron (Unio) chattanoogaensis Lea, Syn., 1870, p. 40. 
Pleurobema chattanoogaensis StMPson, Syn., 1900, p. 753- 
Although this approaches P. decisum so closely in some cases 
that it is difficult to separate the two, I am inclined to believe 
them distinct. This is a more distinctly elliptical shell at al! 
ages than decisum, the beaks are not placed so far forward, 
the anterior end, though sometimes truncate, is cut away little 
if any below, the posterior end is not usually pointed but round, 
and the growth marks are stronger and greener than in that 
species. I am doubtful whether this form is found at Chat- 
tanooga, Tennessee, as Dr. ea was informed. It is probably 
confined to the drainage of the Alabama River system. Ac- 
cording to T. H. Aldrich the animal of this and allied species 
is a brilliant scarlet. 
