THE NAUTILUS. 29 



terior. Outer surface without sculpture. Epidermis lighter 

 or darker brownish, with rather indistinct rays, which are 

 narrower or wider, and do not break up into blotches. Beak 

 sculpture distinct, consisting of rather numerous (six to 

 eight), rather crowded, subconcentric ridges, which form an 

 indistinct, rounded angle upon the posterior ridge, and are 

 in front of this somewhat wavy and corrugated, but without 

 showing any distinct zigzag pattern. Toward the disk, they 

 disappear. Nacre whitish or pinkish. 



Soft parts more or less orange. Anal separated from the 

 supraanal by a well-developed mantle connection, which is 

 shorter than the anal. Anal with small, but distinct papillae, 

 branchial with somewhat larger papillae. Inner lamina of 

 inner gills free. Only the outer gills are marsupial in the 

 female, when gravid, they swell but little, and the placentae 

 are 'subcylindrical (not compressed and lanceolate), rather 

 solid, and of red color. Glochidia semielliptical, of medium 

 size, without hooks. 



Type : Unio siibplanus Conrad. 



This genus stands near Pleurobema and Elliptic, and differs 

 from either chiefly by the subcylindrical, red placentae, and 

 by the beak sculpture. The placentae resemble much those of 

 Fusconaja, but this genus has all four gills marsupial, and the 

 beak sculpture is much more simple. In the beak sculpture, 

 Lexingtonia is peculiar, and might even be said to approach 

 Rotundaria, The general shape of the shell much resembles 

 that of Fusconaja nibiginosa (Lea). Lexingtonia apparently 

 is a collective type, uniting characters found in several other 

 genera, with one character of its own (beak sculpture), and 

 thus the best way out of the difficulty is to create a new genus, 

 which stands between Fusconaja on the one side, and Pleuro- 

 bema and Elliptic on the other. 



LEXINGTONIA SUBPLANA (Conrad). (Unio subplanus Con- 

 rad, Monogr. Union. 9, 1837, p. 73, pi. 41, f. 1, from " branch 

 of James River" (== North Eiver), Lexington, Rockbridge 

 Co., Va. Simpson, Pr. IT. S. Mus., 22, 1900, p. 720: "North 

 Carolina and Virginia"). 



I found seven specimens of this species. One was found at 



