﻿490 s>M I'lnxuTA 



Margaritana rngosa Calkins, i'r. Ottawa Ac. Sci., 1874, p. 40. 

 J.asmigona rugosum RAFiNESQUii:, Cont. Mon., 1831, p. 5. 

 Unio nigosKs KusTiCR, Conch. Cab. Uiiio, 1861, p. 200, pi. lxvi, 



figs. 1-3. — Sovvkkhy, CoTicli. Icon., XV'I, 1867, pi. lx, fig. 



302. 

 JlasnioJoiita hians Ferussac, Guer. Mag., 1835, p. 25. 



\ ery variable in form, sculpture, color, and solidity. It may 

 almost always be separated from all other species at a glance 

 by the peculiar curved, rounded, corrugated ridges of the pos- 

 terior slope, but a specimen before me from Cincinnati, Ohio, 

 is almost smooth in this region, having only a few low, faint 

 nodules. 



The description of Alasinidonta costata of Rafinesque so 

 clearly covers the species described three years later as Alas- 

 modonta rngosa by Barnes that the former name must be used. 



Subgenus Pterosy(jXa Rafinesque, 1831. 



Pterosygna Rafinesque, Cont. Monog., 1831, p. 5. 



Shell large, ovate, rhomboid, inflated in post-basal region : 

 beaks much compressed, their sculpture sharply and strongly 

 doubly looped; epidermis dark, scarcely rayed; teeth very 

 heavy. 



Animal with an extremely thick niarsui>iuni ; palpi large ; 

 mantle with small crenulations on lower border ; branchial 

 opening large ; anal opening small, both slightly papillose. 



Type. Alasmodonta comf>lanata Barnes. 



Svmpiiynota coMin^AXATA ( Panies ) . 



Shell large, solid, irregularly elliptical, obovate or subrhom- 

 boid, inequilateral, compressed : lieaks low, decidedly flattened, 

 their sculpture consisting of coarse, doubly-looped, subnodulous 

 ridges with radiating raised threads behind ; post-dorsal part 

 of the young shell produced into a high, often angular wing, 

 which is tnmcated behind making the outline at this period of 

 growth somewhat triangular : as the shell becomes old this 

 wing breaks off and is worn away ; posterior ridge low, some- 

 times double and ending behind in a biangulation on the medi- 



