UNITED STATES AND ARCTIC AMERICA. 193 



perplexus, (nobis,) but, on looking over his specimens and examining them closely, 

 he had " come to the conclusion that the shell in question is not a perplexus;' and 

 he says, " it is common in the Wabash River, and found on the sand and gravel 

 bars, but never in the mud. On the examination of the mature specimens, I no 

 longer had any doubts of its being distinct from perplexus and Rangianus, to both 

 which it is closely allied in outline. It may at once be distinguished from the former 

 by its being devoid of tubercles, and from the latter by being more robust, more in- 

 flated and in being more quadrate. In the young state it closely resembles also 

 sulcatus, (nobis,) being covered like that species with green capillary rays. Usually 

 these rays are not so strong on the anterior portion as on the middle and posterior 

 portion. It reminds one also of turgidulus, (nobis,) but it has not the angular umbo- 

 nial slope, and the outline differs. The female has an expansion at the posterior mar- 

 gin more like that of Rangianus than perplexus. The undulations of the tips of the 

 beaks are so small and imperfect as only to be observed with the microscope. I have 

 great pleasure in naming this species after Mr, James Sampson, who has clone so 

 much to develope the natural history of the portion of Indiana where he resides. 



Unio macrodon. PI. 26, fig. 262. 



Testa ltevi, triangulari, compressa, subsequilaterali, postice angulata; valvulis suberassis; natibus sub- 



elevatis, ad apices minute undulatis ; epidemiide luteola, striata ; dentibus cardinalibus permagnis, 



subcompressis, elevatis, obliquis erenulatisque ; lateralibus longis rectisque ; niargarita argeutea et 

 iridescente. 



Shell smooth, triangular, compressed, nearly equilateral, angular behind ; valves 

 rather thick ; beaks somewhat raised, minutely undulate at the tips ; epidermis 

 yellowish, striate ; cardinal teeth very large, somewhat compressed, raised, oblique 

 and crenulate ; lateral teeth long and straight ; nacre silvery white and iridescent. 

 Proc. Acad. Nat. Sci., 1859, p. 154. 



Hob. — Rutersville, Texas, Prof. C. G. Porshey. 



Cabinet of Smithsonian Institution. 

 Diam. -5, Length 1, Breadth P7 inch. 



Shell smooth, triangular, compressed, nearly equilateral, angular behind and 

 obliquely rounded before ; substance of the shell rather thick ; beaks somewhat 

 raised, with a few very small undulations at the tips ; ligament rather short and 

 thick ; epidermis yellowish and striate ; umbonial slope slightly raised and some- 

 what angular ; posterior slope narrow and but slightly raised ; cardinal teeth double 

 in the left valve, very large, somewhat compressed, raised, oblique, crenulate, in the 

 right valve single, not so large and more compressed ; lateral teeth long, straight and 

 rather thickened towards the end; anterior cicatrices distinct and deeply impressed; 

 jjusteriur cicatrices distinct and well impressed ; dorsal cicatrices deeply impressed, 



