44 Trans. Acad. Sci. of St. Louis. 



Survey of Michigan, 1861, p. 240. From the Huron 

 river. 



Arkansas specimens have been seen only from the St. Fran- 

 cis, at Wittsburg, and the Saline, at Benton. It doubtless 

 occurs in nearly all the large streams in that State. 



Lamarck's description of Unio rectus runs as follows: — 



** U. testa transver Sim elongata, angusta, convexa, anterius, 

 suhangulata; latere antico striis longitudinalibus ohliquis, 

 remotis ohsoletis. * * * Habite le lac Erie, Michaud. 

 Mile a pi^esque la forme du mytilus lithophagus^ Son test 

 est blanc, reconvert d' un epiderme brun noiratre. Largeur, 

 100 7nillimetres.'' 



The figures here given in the plate are of shells collected 

 in the Des Moines river, and upon similar shells the descrip- 

 tion of the species, given below, is based : — 



Shell large, smooth, elongate, compressed laterally, thick, 

 very thick anteriorly, rounded before, pointed posteriorly, 

 epidermis thick black, or reddish corneous, shining, obscurely 

 rayed with dark green, the broad rays not very apparent in 

 old specimens, lines of growth numerous, well marked, imbri- 

 cated posteriorly, and often so on ventral margin ; dorso- 

 posterior margin straight, or nearly so ; posterior umbonal 

 slopes gently rounded, becoming more angular near the beaks, 

 much imbricated towards posterior margin ; umbones small, 

 scarcely approximating, marked, in non-eroded specimens, by 

 many minute, fine, concentric crenulations or folds; ligament 

 long, thick, black, sometimes dark brown; cardinal teeth 

 double in the left, and disposed to be double in the right valve, 

 the larger portion erect, sometimes sharp, sometimes blunt, 

 and commonly gently posteriorly recurved, the characters of 

 the double portion in left valve various, sometimes sharp, 

 often blunt, or smooth rounded; lateral teeth long, lamellar, 

 straight, finely crenulate on margins ; anterior cicatrices very 

 large, deep, striate, distinct, that of i\iQ protractor pedis im- 

 pression considerably above the lower margin of the large and 

 deep adductor cicatrix ; posterior cicatrices not deeply im- 

 pressed, confluent, very large , pallial cicatrix very deep 

 anteriorly, irregularly impressed throughout, crenulate ; dorsal 

 cicatrices impressed deeply in the center of the cavity of the 



