﻿AI.ASMTDONTA 509 



high above it ; surface with strong, uneven growth Hues, cov- 

 ered with a dull epidermis, which is greenish and shghtly 

 rayed in young shells; in the adult state it is apparently black, 

 but the rays can be seen through it when it is held up to the 

 light ; hinge line sinuous ; left valve with a long, compressed, 

 imperfect tooth at the beak, which is partially divided ; right 

 valve with one compressed, triangular tooth : laterals almost 

 obsolete ; beak cavities very deep ; nacre bluish, bluish-white 

 and thickened in front. 



Length 80, height 60, diam. 50 mm. 



Altamaha River, Georgia. 



Type locality, Altamaha River, Liberty Co., Ga. 

 Margarita (MargaritanaJ arcnla Lka, Syn., 1836, p. 43; 1838, 



p. 27. 

 Margaritaiia arcnla Lka, Tr. Am. Phil. Soc, \'L 1838, p. 71, 



pi. xxii, fig. (k>; O'bs., n, 1838, p. 71, pi. xxii. fig. 69.— 



Chenu, Man., 1859, II, p. 144, fig. 712. 

 Unio arcnla Hani^ey, Biv. Shells, 1843, P- -lO- P'- -"^xii, fig. 49. 



— SowERBY, Conch. Icon., X\'L 1866, pi. xlniii, fig. 259. 

 Margaron (Margaritana) arcula Lea, Syn.. 1852, p. 42 ; 1870, 



p. 67. 

 Alasuwdoiita arcnla Conkad, Pr. Ac. X. Sci. Phila., VL 1853, 



p. 262. 

 Baphia arcnla H. and A. Adams, Gen. Rec. Moll., H, 1857, p. 



500. 

 Margaritana (Alasmodouta) arcnla C1.KSSIN, Conch. Cab. 



Ano., 1876, p. 2y2, pi. i.xxxiii, figs. 7, 8. 

 Alasinidonta-arcnla Simpson, Syn.. i()oo. p. 672. 



.\ remarkable form, which cannot be mistaken for any other. 

 If the character of the longitudinally folded gravid outer gills 

 is constant the species should have generic rank, but I am not 

 so sure that it is. A. triangnlata seems to stand between it and 

 the other Alasmidontas. 



Alasmidonta triangui.ata (Lea). 



Shell short, subtriangular. inflated, rather thin, slightly in- 

 equilateral : beaks high and full, their sculpture a number of 

 strong, concentric ridges with radial lirre in front of and be- 



