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,’ 
3 
which stands appar- 
ently for Tennessee River. 
24. LEXINGTONIA DOLABELLOIDES CONRADI (Vanatta), 1915. 
Unio maculatus Conrad, ’35.—Pleurobema maculatum Goodrich, ’13, 
p. 94.—Pleurobema maculatum Simpson, 714, p. 737.—Pleuro- 
bema appressum Simpson, *14, p. 747.—Pleurobema 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. appresswm (’14, p. 747) undoubtedly 
is this, since he quotes Sowerby’s figure of U. argenteus (Conch. 
icon. 16. Unio. ’66, pl. 37, f. 204), which is a fine representation of 
this form. 
This variety often resembles very much Plewrobema 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, subcylindrical placentee. 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 Ciinton, Anderson Co., Tenn., intergrading 
in the lower part with typical dolabelloides. It is everywhere in the 
