﻿58 i,.\Mi'Si[js 



these rays are sometimes scarcely perceptible and at other times 

 they break into crescentic or arrow-head spots ; left valve with 

 two delicate, subcompressed pseudocardinals placed just in 

 front of the beak, the anterior g-enerally the higher, and two 

 distant, rather short, delicate laterals, the inner the higher; 

 hinge line narrow and rounded in the middle ; right valve with 

 a somewhat compressed pseudocardinal, with a smaller one 

 above it separated by a deep, parallel-sided pit, with often a 

 vestige of another tooth behind, and a single lateral decidedly 

 truncate posteriorly ; beak cavities not deep ; muscle scars shal- 

 low, smooth ; nacre whitish, purple tinted, buff-colored in the 

 cavity of the shell. The female shell is s'horter ajid higher than 

 that of the male and strongly inflated post-basally, the low 

 posterior ridge ends in a decided point about three-fifths of the 

 way up from the base ; the male shell is more blunt or even 

 widely rounded or subtriangulate behind. 



Length (male) 47, height 27, diam. 17 mm. 



Length (female) 44, height 31, diam. 15 mm. 



Type locality. Current River and tributaries, Jack's Ford, 

 and Big Creek, Shannon Co., Missouri. Also, Arkansas. 

 Unio brevicuhis Call, Pr. U. S. Nat. Mus., X, 1887, p. 499, 



pi. XXVIII ; Tr. Ac. Nat. Sci., St. Louis, VII, 1895, p. 6, pi. 



xvn. 

 Lampsilis hreviculus Simpson, Syn., 1900, p. 533. 



I cannot be certain as to the relationships of this species. 

 The female is a good deal like a small, very delicate L. cariosa, 

 but the tint of color is softer, especially that of the nacre, 

 which is silvery, and the rays are different. 



Var. brittsi Simpson. 



Shell more elongated than the type, with deeper sinus be- 

 hind the marsupial swelling and more distinctly rayed. 

 Lampsilis brittsi Simpson, Pr. Ac N. Sci. Phila., 1900, p. 76, 



pi. V, figs. I, 2. 

 Lampsilis breviculus var. brittsi Simpson, Syn., 1900, p. 533. 



Quite different from the type but rather variable, and in 

 deference to the opinions of some of our best conchologists I 

 have reduced it to the rank of a varietv of brez-iciila. 



