546 ORTMANN— NAYADES OF 



It is remarkable that this form has not been found in the Holston- 

 proper, from Knoxville up to the forks, while the headwaters-form 

 is both in the North and South Fork Holston. 



Type locality: *' Holston River, Tennessee," which stands appar- 

 ently for Tennessee River. 



24. Lexingtonia dolabelloides conradi (Vanatta), 1915. 



Unio maculatus Conrad, '35. — Pleurohcma maculatuni Goodrich, '13, 

 p. 94. — Pleurohcma maculatum Simpson, '14, p. y^J.—Plcuro- 

 bema appressum Simpson, '14, p. 747. — Pleurohcma conradi 

 Vanatta, '15, p. 559. 



A form, largely misunderstood. Specimens of this have often 

 been called U. appressus Lea, because two specimens are in the U. S. 

 Nat. Mus. with the figured type of appressus, but are different from 

 the latter. Also Simpson's P. appressum ('14, p. 747) undoubtedly 

 is this, since he quotes Sowerby's figure of U. argenteus (Conch, 

 icon. 16. Unio. '66, pi. 37, f. 204), which is a fine representation of 

 this form. 



This variety often resembles very much Pleurohcma oviforme 

 (Conr.) (chiefly such forms which have been called clinchense 

 Lea). However, it may be recognized by the subtriangular outline, 

 more forwardly inclined beaks, and the more distinct truncation of 

 the posterior slope. The most important character, however, is seen 

 in the soft parts : in the present form, these are generally deep 

 orange, with the outer gills marsupial, filled, when charged, with 

 red, sub cylindrical placentae. These are the characters of the genus 

 Lexingtonia. The main species has the same characters, at least in 

 the Clinch; in the French Broad, however, I have found specimens 

 with pale soft parts. But the same tendency has been noticed in 

 other species from the French Broad. 



An abundant form in the headwaters of Powell, Clinch, North 

 and South Fork Holston. In the Powell, it goes up to Big Stone 

 Gap, Wise Co., Va., and is also in Puckell Creek at Pennington Gap, 

 Lee Co., Va. In the Clinch, it is found up to Cedar Bluff, Tazewell 

 Co., Va., and down to Clinton, Anderson Co., Tenn., intergrading 

 in the lower part with typical dolahcUoides. It is everywhere in the 



