155 



very deep : palleal impression emarginate opposite to tlie denticu- 

 lated angle : above this angle also, tlie concavity is abruptly more 

 concave, so as to form a definite dilated groove. 



Var. a. Within deep reddish purple. 



Obs. 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 wrinkles. 

 [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 sulcafus* 

 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 fi^ssure. 



I obtained numerous specimens at the Falls of the Ohio, 'on the 



* [I change y?a5re?/a^us to sulcatus, in accordance with an erratum of the 

 author. — Ed.] 



