﻿ALASMinoXl'A 505 



the ridge there is a decided oblique truncation and the trun- 

 cated area is rather nnely corrugated ; growth Hues uneven ; 

 epidermis smooth, shining, greenish, ashy or smoky-green 

 with narrow yellowish rays and broken, clouded green ones ; 

 left valve with one small, compressed, sharp tooth in front of 

 the beak ; right valve with a small anterior tooth ; laterals 

 almost or quite wanting ; beak cavities rather deep ; nacre 

 white or bluish, sometimes straw-color, rarely reddish or pur- 

 ple ; prismatic border distinct. 



Length 120, height 72, diam. 48 mm. 

 Length 109, height 58, diam. 45 mm. 



Upper Mississippi drainage ; Ohio, Cumberland, and Ten- 

 nessee River systems ; Michigan ; Upper St. Lawrence drain- 

 age. 



Type locality, Scioto River. Chillicothe, O. 

 Alasmodonta marginata Say. Nich. Encyc, 1819, No. i ; Jl. 

 Ac. Nat. Sci. Phila.. L 1819, p. 459. — Rafinesoue, Ann. 

 Gen. Sci. Brux., V, 1820, p. 317. 

 Margarita (Margaritana) marginata Lea, Syn., 1836, p. 43;; 



1838, p. 27. 

 Margaron (Margaritana) marginata Lea, Syn., 1852, p. 42;. 



1870, p. 67. 

 Margaritana marginata Kuster, Conch. Cab. Unio, 1862. p.. 



297, pi. xcix, fig. 3. 

 .Alasmodonta marginata Baker, Moll. Chicago, Pt. i, 1898. p. 



62, pi. IV, fig. 4 ; VII, fig. 7 ; XXII. fig. 3. 

 Margaritana marginuta var. truncata B. H. Wrtght, Naut., 



XL 1898, p. 124. 

 Unio marginatiis Sowfrp-v, Conch. 'Icon., X\T, 1866, pi. Li, 



fig. 267. 

 Alasmodonta truncata Conrad, Pr. Ac. Nat. Sci. Phila.. \ 1, 



1853, p. 262. 

 Alasmidonta truncata Simpson, Syn., 1900. p. .^71- 



In rare instances the beaks of this species are as near to or 

 nearer to the posterior end than to the anterior. Occasionally 

 the tooth in the left valve is divided and both are often a little 



