Illustrations of Little Knoivn Unionidce. 



1 59 



" Largeur 2 a 3 pouces ; test plus epais anterieurement, a grosses 

 rides et a quelques tubercules oblongs ; une grosse bosse oblique lon- 

 gitudinale; dents epaisses striees. Aux chutes de l'Ohio." 



Rafinesque did not attempt to figure this species, as he had done 

 with most of the shells described in his memoir. In the absence of 

 a good figure only his technically imperfect and brief description may 

 be relied on to furnish a clue to the shell he intended in this case. 

 There has never been much question in my mind that he had Unto 

 cesopus before him when he made his diagnosis. No other Ohio River 

 Unto could possibly satisfy the characters here given. The single 

 character of " chestnut brown epidermis '* alone is not true of most 

 specimens of this form, but such examples are often found. The 

 original description is here repeated that the reader may himself decide 

 whether a careful review of the species will not credit the form to 

 Rafinesque. 



In pursuance of the objects of this series of papers a new descrip- 

 tion is here given, based upon recent and well-preserved specimens 

 from the Ohio. The plate given is based upon a specimen from the 

 Des Moines River, Iowa, and is of a full-grown male and a young 

 female. 



Unio /Esopus, (ireen. (Descriptio emend.) 



Shell sub-oval, somewhat flattened laterally, incrassate anteriorly, 

 thinner over posterior region, obscurely tuberculate from middle of 

 umbonal slope to ventral margin, the obtuse tubercles being alternately 

 disposed and usually largest about the middle of the disk ; the an- 

 terior margin is regularly rounded as far as the region of the umbonal 

 tubercules, then usually emarginate or sulcate, a character which is 

 particularly well marked in the old female and the very young ; growth 

 striae numerous, crowded, and conspicuous, the lines indicating arrest 

 of growth being darker and broader, well marked and rather deeply 

 impressed; ligament light brown, smooth and flat-elliptical: lunule 

 evident and somewhat cordate : the beaks are coarsely and concen- 

 trically undulate, the undulations numbering three or four, with the 

 epidermis somewhat lighter in color than that on the slopes of the 

 umbones; cardinal teeth single in the right, and double in the left 

 valve, triangularly pyramidal ; lateral teeth lateral in the left, and dis- 

 posed to be double in the right valve, nearly or quite straight in old 

 specimens, or slightly curved downward in the very young shell, 

 thick; anterior cicatrices distinct, pit-like and deeply impressed; pos- 



