548 ORTMANN— NAYADES OF 



Clinch, Holston, and French Broad. It goes up, in the CHnch, to 

 Union Co., Tenn. ; in the Holston, to Grainger Co., Tenn, In the 

 Tennessee proper it is known down to Chattanooga. It is rather 

 strange that it is absent in Lewis's list. 



In specimens from the upper section of Holston and Clinch, the 

 radial furrow is quite shallow. 



Type locality: Ohio River. 



26. Pleurobema obliquum cordatum (Rafinesque), 1820. 



Ohovaria cordata Rafinesque, '20. — Unio plenus Lea, '40. — Unio 



plenus Lewis, '71. — Quadrula plena Simpson, '14, p. 886. — Quad- 



riila cordata Vanatta, '15, p. 558. — Pleurobema obliquum plenum 



Utterback, '16, p. yy. 



I accept the nomenclatural change introduced by Vanatta, since 

 Rafinesque's description and figure can very well be referred to this 

 form. 



Upright, more rounded, and more elevated than the normal form, 

 radial furrow less developed. 



This is the most poorly marked form of the group, and is found 

 with the main form all over its range, representing merely an indi- 

 vidual variation. In the upper Tennessee region it is rather scarce. 



Type locality: Ohio River. 



27. Pleurobema obliquum catillus (Conrad), 1836. 



Unio catillus Conrad, '36. — Unio solidus Lea, '38. — Q.uadrula solida 



Simpson, '14, p. 885. — Pleurobema obliquum catillus Utterback, 



'16, p. 79. 



Subtriangular, rather swollen, with the radial furrow obliterated 

 or absent. Nacre white or reddish. 



Individual variation of the main form, all over its range, but quite 

 rare in the upper Tennessee ; there are mighty few specimens which 

 show the characters of this form well developed. 



In other regions (upper Ohio, and west of the Mississippi) this 

 form assumes frequently the character of a local race, in fact, west 

 of the Mississippi, this, and forms like P. obliquum rubrum, prevail, 



