550 ORTMANN— NAYADES OF 



29. Pleurobema obliquum rubrum (Rafinesque), 1820. 



Ohiiquaria rubra Rafinesque, '20. — Unio pyramidatus Lea, '34. — 

 Unio mytiloideshe\vis,'yi. — Unio pyramidatus Pilsbry & Rhoads, 

 '96. — Pleurobema pyramidatum Ortmann, '12&, p. 264 (anatomy). 

 — Quadrula pyraiiiidata Simpson, '14, p. 888. — Quadriila rubra 

 Vanatta, '15, p. 557. 



According to Vanatta, the type of Obliquaria rubra Raf. is the 

 same as U. pyramidatus of Lea. Since ah-eady Conrad ('34 and '36) 

 has brought rubra together with pyramidatus Lea and his own 

 mytiloidcs (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 anatomw and 



