UPPER TENNESSEE DRAINAGE. 547 
North Fork Holston up to Saltville, Smyth Co., Va., and in the 
South Fork Holston at Fish Dam (Walker coll.), Emmett, and Bluff 
City, Sullivan Co., Tenn. However, it does not go down the Holston 
proper (see above). 
Type locality: (of U. maculatus Conr.) Elk and Flint rivers, 
northern Alabama (Conrad’s Flint River is in Morgan Co., Ala., 
and is different from the Flint River in Madison Co., Ala.). 
Note: This group of forms is also abundant in the Tennessee 
drainage in northern Alabama. The typical dolabelloides is found 
in the Tennessee proper, and also in lower Paint Rock River, Flint 
River (Madison Co.), and Limestone Creek; while the conradi-type 
is found in smaller streams, for instance, in the headwaters of Paint 
Rock River, the headwaters of Flint River (Madison Co.), Flint 
River (Morgan Co., according to Conrad), and in Elk River 
(Conrad). 
Genus: PLEUROBEMA Rafinesque (1820). 
Ortmann, 19120, p. 261. 
25. PLEUROBEMA OBLIQUUM (Lamarck), 1819. 
Unio obliqua Lamarck, ’19 —Unio obliquus Pilsbry & Rhoads, ’96.— 
Pleurobema obliquum Ortmann, ’12b, p. 264 (anatomy ).—Quad- 
rula obliqua Simpson, 714, p. 881. 
This consists of a group of forms very variable in shape, which 
has been divided into a number of “species.” In the upper Ten- 
nessee region several of the latter are found, but they all intergrade 
with each other, and there is very little indication of their separation 
into geographical or ecological races. Mostly, the various forms are 
found associated, so that they are hardly more than individual 
variations. 
However, in deference to the nomenclature hitherto accepted, 
and in view of the fact that in other regions some of these variations 
become local varieties, I have kept these forms apart. 
The typical P. obliquum is rather upright, with a distinct radial 
furrow, and is subtriangular in outline. The nacre is generally white. 
This form is quite abundant in the larger rivers, Tennessee, 
