550 ORTMANN—NAYADES OF 
29. PLEUROBEMA OBLIQUUM RUBRUM (Rafinesque), 1820. 
Obliquaria rubra Rafinesque, ’20.—Unio pyramidatus Lea, ’34.— 
Unio mytioides Lewis,’71—Unio pyramidatus Pilsbry & Rhoads, 
’96.—Pleurobema pyramidatum Ortmann, ’12), p. 264 (anatomy ). 
—Quadrula pyramidata Simpson, ’14, p. 888.—Quadrula rubra 
Vanatta, ’15, p. 557. 
According to Vanatta, the type of Obliquaria rubra Raf. is the 
same as U. pyramidatus of Lea. Since already Conrad (’34 and ’36) 
has brought rubra together with pyramidatus Lea and his own 
mytiloides (which is pyramidatus), this nomenclatural change should 
be accepted. 
Shell oblique, with high, forwardly inclined beaks. Radial fur- 
row more or less developed. Nacre generally red. 
Although, in its typical phase, quite distinct from normal obli- 
quum, intergrades do exist, and also intergrades toward cordatum 
and catillus have been observed. 
Also this form generally accompanies P. obliquum, but both in 
Clinch and Holston it ascends the rivers a little farther. In the 
Clinch, it goes to Oakman, Grainger Co., Tenn., and in the Holston 
to Austin Mill, Hawkins Co., Tenn. It is abundant and well devel- 
oped in this region, and of all the forms of the obliquum group, it 
has the best claim to be regarded as a local race. 
In the upper Ohio, this form is quite rare, and often not typical. 
West of the Mississippi are peculiar forms of it, often with the radial 
furrow quite obliterated. 
Type locality: Kentucky River. 
GROUP OF PLEUROBEMA OVIFORME. 
Also here we meet with a group in which the rule holds good 
that flat forms of the headwaters are represented, farther down 
stream, by more swollen forms. 
Generally speaking, the oviforme group represents, in the upper 
Tennessee, the P. clava of the Ohio drainage, but it is much more 
variable, and has developed, in the headwaters, a very peculiar, com- 
pressed type, which does not find a parallel in the upper Ohio system. 
All these forms have the characteristic Pleurobema anatomy, and 
