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. macnlatus 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, 19121(7, p. 261. 



25. Pleurobema obliquum (Lamarck), 1819. 



Unio obliqua Lamarck, '19. — Unio obliqiiiis Pilsbry & Rhoads, '96. — 

 Pleurobema obliquum Ortmann, '12b, p. 264 (anatomy). — Quad- 

 rnla obliqua Simpson, '14, 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, 



