﻿HKMII.ASTENA 325 



All four of the gills are united to the mantle behind to their 

 extreme ends, while in all species of Margaritana they are free 

 behind. 



Hemilastena ambigua (Say). 



Shell small, oblong, with dorsal and basal lines parallel, 

 the anterior and posterior ends rounded, inequilateral, sub- 

 solid, not inflated; beaks not full or elevated, their sculpture 

 consisting of fine ridges looped up in the middle and open 

 behind ; posterior ridge wide and rounded, often raised above 

 the disk ; surface with delicately sulcate growth lines ; epider- 

 mis greenish or olive-brown; there is a single low, smooth 

 tooth in each valve at the beaks ; nacre thin behind, where it 

 is subtranslucent and tinted with dirty purplish ; at the anterior 

 base it is thickened and creamy white. 



Length 40, height 19, diam. 12 mm. 



Length 48, height 21, diam. 16 mm. 



Ohio River system; north to Michigan; west to Iowa; south 

 to Arkansas ; east to Tennessee. 



Type locality. Northwestern Territory. 



Alasmodonta ambigua Say, Jl. Ac. N. Sci. Phila., V, 1825, 

 p. 131. 



Margaritana ambigua Kuster, Conch. Cab. Unio, 1862, p. 300, 

 pi. xcix, fig. 7. 



Hemilastena ambigua Simpson, Syn., 1900, p. 673. 



Unio hildrcthianus Lea, Tr. Am. Phil. Soc, V, 1834, p. 36, pi. 

 Ill, fig. 8 ; Obs., I, 1834, p. 148, pi. Ill, fig. 8.— Hanley, Biv. 

 Shells, 1843, P- 196, pl. xxiii, fig. 38.— KusTUR, Conch. Cab. 

 Unio, 1861, p. 206, pl. i,xviii, fig. 8.— SowERBY, Conch. 

 Icon., XVL 1866, pl. xxxvi, fig. 192.— Baker, Moll. Chi- 

 cago, Ft. I, 1898, p. 71, pl. v,X\g. 4. 



Margarita (Unio) hildrethianus Ul\, Syn.. 1836, p. 28; 1838, 

 p. 20. 



Margaron (Margaritana) Jularethianus Lea. Syn., 1852, p. 

 43 ; 1870, p. 69. 



Strophitus hildrethiana Conrad, Pr. Ac. N. Sci. Phila., VI, 

 1853, p. 263. 



