﻿AI.ASMIDONTA 499 



] think it more than Hkely that this will prove only a mere 

 variety or local race of A. calceolus, when a sufficient amount 

 of material is examined. It seems to be quite rare, and I have 

 never been able to see more than a few shells from the Tennes- 

 see and Cumberland systems. The posterior ridge is a little 

 lower and more rounded than in A. calceoliis and the posterior 

 point is more widely rounded ; the epidermis is a little differ- 

 ently colored, the lighter part of it being yellowish instead of 

 ashy. 



Group of Alasmidonta heterodon. 



Shell small, rather thin, rhomboid ; beak sculpture very 

 strong, consisting of concentric ridges or irregular corruga- 

 tions, often slightly doubly looped and swollen on the well- 

 developed posterior ridge ; surface rayed ; pseudocardinals 

 compressed, reflexed ; laterals single, double, or triple in each 

 valve. 



Animal with branchial opening having a few coarse papill?e ; 

 anal opening smooth, or with very fine crenulations. 



Alasmidonta heterodon (Lea). 



Shell small, long rhomboid, inequilateral, convex, thin to 

 subsolid : beaks low but rather full, their sculpture consisting 

 of a few strong, corrugated ridges, which run nearly parallel 

 with the growth lines, with radiating threads in front of and 

 behind them ; posterior ridge well developed, narrowly round- 

 ed or subangular, ending in a rather sharp point a little above 

 the base : surface with uneven growth lines, dirty greenish, 

 faintly rayed, dull colored ; left valve with two pseudocar- 

 dinals. the hinder somewhat broken and compressed ; right 

 valve with one compressed pseudocardinal ; laterals imperfect, 

 lamellar, one or two. sometimes three in each valve ; nacre 

 l)luish-white. 



Length 36, height i8, diam. 12 mm. 



iVorthern New England to Virginia. 



'J'ype loc.ility Schuylkill River and Derby Creek, Pa. 



