CHIEFLY OF THE UNITED STATES. 311 



ligament short, thin and very dark brown ; epidermis black, subsquamose, without 

 rays, with close marks of growth; umbonial slope rounded, sometimes with a raised 

 line ; posterior slope elliptical, with a slight furrow on each valve from the beaks to 

 the posterior margin; cardinal teeth very small and decussate; lateral teeth long 

 and somewhat straight; anterior cicatrices distinct, small and well impressed; 

 posterior cicatrices confluent, very slightly impressed; dorsal cicatrices placed in 

 the centre of the cavity of the beaks ; cavity of the shell rather deep and wide ; 

 cavity of the beaks shallow and obtusely angular; nacre white and iridescent. 



Remarhs. — I have had most of the four specimens of this little black species, now 

 before me, for many years, always doubtful as to whether they might belong to 

 parvus, Bar., or to paulus (nobis). Now, after a thorough examination and compari- 

 son, I am convinced that it cannot belong to either of these two species, and therefore 

 must be considered to be a new one. The group of little shells of which this is a 

 member are all very much alike, while there exists a difference which the practised 

 eye easily analyzes. This species has a well-pronounced angle on the umbonial slope, 

 which is absent on the other species. The epidermis is more rough and blacker than 

 the others. All the four specimens have a pure white nacre, and the beaks of these 

 are all eroded, so that we must remain ignorant if there be any undulations, until 

 perfect specimens are found. 



Unio difficilis. pi. 49, fig. 124. 



Testa Itevi, ellii^tica, inflata, valde inssequilaterali, postice obtuse angulata, antice rotundata ; valvulis 

 tenuibus, antice crassioribus ; natibus prominulis ; epidermide luteola, valde radiata ; dentibus 

 cardinalibus parviusculis, conicis crenulatisque ; lateralibus sublongis rectisque ; margarita alba 

 et valde irideseente. 



Shell smooth, elliptical, inflated, very inequilateral, obtusely angular behind, 

 rounded before ; valves thin, thicker before ; beaks a little prominent ; epidermis 

 yellowish, very much radiated ; cardinal teeth rather small, conical and crenulate ; 

 lateral teeth rather long and straight ; nacre white and iridescent. 

 Proc. Acad. Nat. Sci. 1868, p. 144. 



Eah. — Swamp Creek, "Whitfield Co., Geo., Maj, Downie, and head-waters of the 

 Holston, Washington Co., Va., Prof. Cope. 



My cabinet and cabinet of Academy of Natural Sciences. 

 Diam. -6, Length -9, Breadth 1-5 inch. 



Shell smooth, elliptical, inflated, very inequilateral, obtusely angular behind, 

 rounded before; substance of the shell thin, thicker before ; beaks a little prominent; 

 ligament short and dark brown ; epidermis yellowish, very much radiated, with 

 distant marks of growth ; umbonial slope raised and rounded ; posterior slope ellii> 



