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Art. VIII. — New Exotic Unionidce. 

 By Isaac Lea. 



Most of the species described in this paper are from South America, and nearly 

 the whole of these were collected hy Prof. J. Wyman. A few are from Asia, pre- 

 sented by various friends, to whom they are duly credited. 



During the winter of 1858-59, R. B. Forbes, Esq., of Boston, whose name has 

 been identified with so many works of philanthropy and public utility, organized an 

 excursion to the La Plata, Uruguay and Rio Negro Rivers in South America ; his 

 object in part being to afford facilities for studying the natural history of the 

 countries bordering on those waters. Prof. Wyman, who accompanied him, has 

 most kindly placed at my disposal all the specimens of the Unionidce which he had 

 been enabled to collect in these extensive southern fresh waters. In this very interest- 

 ing collection I was surprised to find so many species which had not been before 

 observed. These are now herein described, and consist of eleven Uniones and four 

 Anodontce. The whole number brought of these fresh water Molluscs was twenty- 

 three species. Those heretofore described are Prisodon trwncatus, Schum., (Castalia 

 arnbigua, Lam.,) Unio Parunensis, Lea, U parallelopipedon, Lea, Anodonta rotunda, 

 Spix, A. trapezalis, Lam., A. lalo-marginata, Lea, A. tenebricosa, Lea, A. Blainvilliana , 

 Lea. In addition there were three small species of Gyrena, two of which I have 

 not ascertained, the third is the vdriegata of d'Orbigny. There was also a small 

 species of Gyclas. 



Unio yethiops. PL 41, fig. 285. 



Testa, Irevi, oblonga,, subinflata, ad latere planulata, valde inasquikterali, postice biangulata, antice 

 rotundata ; valvulis crassiuseulis, anticti erassioribus ; natibus pvotuinulis, planulatis, ad apices 

 divaricate undulatis ; opidermide micante, nigra, striata, eradiata ; dentibus cardinalibus parvi- 

 usculis, ccmipressis, obliquis, subereetis crenulatisque ; lateralibus prrelongis, erenulatis reetisque; 

 niargarita alba et iridescente. 



Shell smooth, oblong, somewhat inflated, flattened at the side, very inequilateral, 

 biangular behind, rounded before; valves somewhat thick, thicker before; beaks 

 somewhat prominent and flattened, with diverging undulation at the tips ; epider- 



90 



