548 ORTMANN—NAYADES OF 
Clinch, Holston, and French Broad. It goes up, in the Clinch, 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.° 
Obovaria cordata Rafinesque, ’20.—Unio plenus Lea, ’40—Unio 
plenus Lewis, ’71—Quadrula plena Simpson, *14, p. 886.—Quad- 
rula cordata Vanatta, ’15, p. 558.—Pleurobema obliquum plenum 
Utterback, ’16, p. 77. 
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—Quadrula solida 
Simpson, ’14, p. 885.—Pleurobema obliquum catillus Utterback, 
sO; Di 70: 
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, 
