112 XEW UNIONID.T? OF THE UNITED STATES. 



Hah. — Tornbigbee River, near Columbus, Mississippi. Win. Spillmnn, M D. 



My cabinet and cabinet of Dr. Spillman. 

 Diam. 1'4, Length, 2, Breadth, 31 inches. 



Shell smooth, elliptical, inflated, obtusely angular behind, rounded before, sub- 

 equilateral ; substance of the shell rather thin ; beaks a little prominent, swollen, 

 rugosely and subconceutrically undulate at the tips ; ligament short, thick and dark 

 brown; epidermis dark olive, sometimes nearly black, obscurely rayed and with 

 very distant marks of growth ; umbonial slope very much raised and very obtusely 

 angular ; posterior slope broad, subcordate, with irregularly impressed lines from the 

 beaks to the margin ; cardinal teeth small, corapressed-tuberculate ; anterior cicatrices 

 distinct, large and slightly impressed ; posterior cicatrices confluent, large and very 

 slightly impressed ; dorsal cicatrices placed on the base of the cardinal tooth ; cavity 

 of the shell very deep and rounded ; cavity of the beaks deep and wide ; nacre white 

 and iridescent. 



Soft parts. — Branchial xiteru-^. — All the four specimens received in alcohol seemed 

 to be males — could find no ova. Brancliim very large, free nearly half the length of 

 the abdominal sack. Palpi small, subelliptical, united only at the upper part of the 

 posterior edges. Mantle very thin, with a broad border. On the posterior exterior 

 edges there is a dark, blackish line, separating occasionally into maculations, a charac- 

 ter so common to the genus Margaritana.* Branchial opening large, blackish, with 

 small, brown papillje on the inner edges. Anal opening rather small, blackish, with 

 very small papillse on the inner edges. Super-anal opening rather small, maculate 

 on the outer edges, united below. Anus crenulate on the rim. Color of the mass 

 whitish. 



Remarlcs. — This is a well marked species not easily confounded with any other. 

 Its proportional length with the transverse diameter is greater than any other of our 

 species of Margaritana, and its beaks are more medial. It is remarkable for its reg- 

 ular elliptical outline and its obtruded anterior mai'gin. The young have green rays 

 over nearly the whole disk. The undulations of the beaks are coarse, and usually 

 consist of four on each valve. The old individuals are much disposed to be imper- 

 fect about the region of the ligament, beaks and teeth. 



Akodonta Kennerlyi. pi. 18, fig. 256. 



Testa Ifevi, elliptica, subcylindracea, subventricosa, valdt; inasquilaterali, antice subtruncata, postice bian- 

 gulata ; valvulis tenuissimis, diaphanis ; natibus vix prominentibus, ad apices exilissim^ undulatis ; epi- 

 dermide luteo-oliva, micante, ad margine striata, eradiata ; margarita cseruleo-alba et valde iridescente. 



Shell smooth, elliptical, subcylindrical, somewhat ventricose, very inequilateral, 

 somewhat truncate before, biangular behind ; valves very thin, semitransparent; beaks 



* One specimen was maculate the whole distance to the upper end of the super-anal opening'. 



