155 
very deep : palleal impression emarginate opposite to the denticu-' 
lated angle : above this angle also^ the concavity is abruptly more 
concave, so as to form a definite dilated groove. 
Var. a. Within deep reddish purple. 
Ohs. The remarkable character of the anterior basal angle or 
lobe of this species, renders an extended and more particular 
description unnecessary. In the figure of the annexed plate, we 
have brought this character particularly into view, by a delineation 
of the front of the shell, and two or three series of the teeth are 
visible in some specimens, like varices in the univalves, on the 
surface of the shell, fringing the deeper undulations or v/rinkles. 
[We may remark that in our figure these teeth have too much of 
a serrated appearance ; they are in reality more tooth-shaped, 
though slender.] It has considerable resemblance to the sulcatus^ 
Lea, in general form, brevity of the posterior side, summits, cavity 
of the hinge membranes and radiated surface, but the denticulated 
anterior edge, the more acute anterior groove and other subordinate 
characters, amply distinguish it. It is a small species, even smaller 
than the sulcatus.'^ 
A remarkable and gigantic variety occurs in Cumberland river; 
a specimen is rather more than two inches wide and the radii are 
interrupted into spots. It may prove to be a distinct species when 
more specimens are obtained for comparison, and it may in that case 
be distinguished by the name of perplexus. For a specimen of 
it I am indebted to Mr. Lesueur. • PI. 5. 
Unio monodonta. — specific character. Transversely elon-= 
gated ; inner margin whitish or livid ; lateral teeth obscure. 
Desc. Shell transversely much elongated, slightly arquated, 
blackish-brown; umbonial slope (Swainson) widely rounded, not 
abruptly elevated : umbo placed far back, not prominent above the 
general curvature ; hinge and basal margins a little arquated, pa~ 
rallel : anterior and posterior margins regularly rounded : within 
perlaceous, with an opake whitish or livid margin : primary teeth 
simple, small : lateral teeth very imperfect ; that of the right valve 
with only an impressed, short line, instead of the usual deep recipi- 
ent fissure. 
I obtained numerous specimens at the Falls of the Ohio, on the 
* [I cliange fiagellatus to sulcatus, in accordance with an erratum of the 
author. — E d.] 
