566 ORTMANN— NAYADES OF 



49. Dromus dromas (Lea), 1834. 



Unio dromas Lea, '34. — Unio dromas Lewis, '71. — Unio dromas 

 Pilsbry & Rhoads, '96. — Dromus dromas Ortmann, '12b, 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: Llarpeth River, Tenn. (and Cumberland River, 

 Nashville, Tenn.). 



50. Dromus dromas caperatus (Lea), 1845. 



Unio caperatus Lea, '45. — Unio caperatus Lewis, '71.- — Dromus 



caperatiis 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 Jeft'erson 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, etc. 



