748 PILEUROBEMA 
rays; left valve with two strong pseudocardinals, which are 
slightly roughened, the posterior slightly divided, with two 
curved laterals, the lower the stronger; right valve with one 
strong pseudocardinal, with a vestigial one in front of it and 
sometimes one or more imperfect denticles behind it and one 
lateral ; hinge plate wide and flat ; cavity of the beaks not deep, 
but compressed; muscle scars small, impressed; pallial line 
crossed by. short radiating ridges; nac:e dirty or lurid white, 
blotched in the type. 
Length 60, height 47, diam. (near anterior end) 28, near 
posterior end 24 mm. 
Clinch and Holston rivers. 
Type locality, Tuscumbia, Ala.; Tennessee River and Hols- 
ton River, Tenn. 
Unio appressus Lea, Pr. Ac. N. Sei. Phila. Ij, 1871; pa 169; 
jl. Ac. Ny Sei: Philaz, VILEA1874; py pe a hes aban 
XIII, 1874, pl. 11, fig. 8. 
Pleurobema appressa SIMPSON, Syn., 1900, p. 749. 
Unio argenteus SowrERBY, Conch. Icon., XVI, 1866, pl. xxxvit, 
fig. 204. 
This species is dangerously near two or three others. The 
fact is an almost unbroken chain seems to exist connecting this 
with Unio tuscumbiensis, favidus, tumescens, radiosus, dolla- 
helloides, thorntonii, mooresianus, recurvatus, circumactus and 
some other forms. Most specimens of appressum are a little 
more compressed than most of these and are tawny-brown 
and rayless, though the type is rayed and more inflated than 
the average. I confess that with the types of all these so- 
called species and an abundance of other material, 1 am unable 
to make any arrangement that is satisfactory. 
PLEUROBEMA TUSCUMBIENSE (Lea). 
Shell subtriangular or subrhomboid, subinflated to inflated, 
solid, inequilateral: beaks high, full, long, their sculpture ap- 
parently a few broken, nodulous ridges ; lunule well developed ; 
anterior end somewhat obliquely truncate above, rounded be- 
low; base line curved or straight; outline of the dorsal slope 
curved, elevated almost to an angle behind the ligament; pos- 
