566 ORTMANN—NAYADES OF 
49. DromMus proMAs (Lea), 1834. 
Unio: dromas Lea, ’34.—Unio dromas Lewis, ’71—Umnio dromas 
Pilsbry & Rhoads, ’96.—Dromus dromas Ortmann, ’12), p. 315 
(anatomy ).—Dromus dromas Simpson, ’14, p. 341. 
The typical form is rather swollen, and has a large knob or hump 
‘on each valve. This knob, however, is very variable. 
This form is found in the Tennessee proper. It has been re- 
ported by Lewis from below Knoxville, and by Pilsbry and Rhoads 
from Chattanooga, Hamilton Co. It is in the Walker collection from 
Chattanooga and from Rathburn, Hamilton Co. Pilsbry and Rhoads 
report it also from the Holston, at Boyd Island, near Knoxville. I 
found it only in the Tennessee, three miles below Knoxville. 
There are occasional specimens, which might be called by this 
name, in the lower Clinch and Holston, but in this region it is gen- 
erally replaced by the next form, with which it intergrades. 
Type locality: Harpeth River, Tenn. (and Cumberland River, 
Nashville, Tenn.). 
50. DROMUS DROMAS CAPERATUS (Lea), 1845. 
Unio caperatus Lea, ’45—Unio caperatus' Lewis, ’71—Dromus 
caperatus Simpson, ’14, p. 343. 
This form has lost the “hump,” and it is more compressed than 
the normal form. It begins in the Tennessee at Knoxville (Lewis), 
and ascends both the Clinch and Holston, where it intergrades, in the 
lower parts, with typical D. dromas. In the Holston, it goes up to 
Holston Station, Grainger Co., Tenn., and in the Clinch, it goes to 
Clinch River Station, Claiborne Co., Tenn. It enters also the Powell, 
and goes up to Shawanee, Claiborne Co., Tenn. 
It is quite abundant in the Holston in Knox and Jefferson Cos., 
although it is frequently associated here with the typical form, and 
intergrades with it. Its metropolis, however, is in the Clinch from 
Anderson Co. upward, and in the Powell, where it is a pure race. 
Type locality: Clinch River (topotypes examined). 
Note: The tendency to develop here a more compressed form in 
the smaller rivers agrees entirely with the similar phenomenon ob- 
served in species of Fusconaia, ete. 
