8 THE NAUTILUS. 



sideration is given to the original description and figure. It is with- 

 out doubt one of the most common species in the upper Mississippi 

 and Ohio drainage systems and is commonly known as Quadrula tri- 

 gona Lea. 



Barnes's description is very accurate and his figures (PI. I, figs. 1 

 & 2) though rude are quite recognizable. As the final determination 

 of the standing of his species must rest upon them, and the publica- 

 tion in which they appeared is out of date and not easily obtained, 

 both are here reproduced, so that they may speak for themselves. 

 In drawing his description, Barnes followed the prevalent error of 

 the day and reversed the ends of the shell, calling the anterior ex- 

 tremity the posterior and vice versa. I have therefore interpolated 

 the proper corrections in this particular in parentheses. The italics 

 are as in the original. 



Unio undatus. 



Shell subtriangular, sub-longitudinal, very tumid, waved ; lateral 

 teeth, two in each valve. 



Unio ohliqua'} M. Lamarck. 



Hab. Ouisconsin and Fox Rivers. Mr. Schoolcraft. 



Diam. 1. 5. Length 2.1. Breadth 2.2. 



Shell thick, disks swelled behind (before), depressed before (be- 

 hind), anterior (posterior) side slightly produced, rapidly narrowed, 

 angulated ; beaks projecting backward (forward) nearly as far as the 

 posterior (anterior) side, elevated and recurved, with the ligament 

 passing between them ; anterior (posterior) lunule long heart-shaped 

 and separated by a slightly elevated heel ; hinge margin depressed 

 between the beaks ; basal margin waved and rounded behind (be- 

 fore), compressed in the middle, angulated before (behind); epidermis 

 hoi r-color exhibiting a light yellowish-green where the surface is 

 worn or rubbed, wrinkled and finely striated transversely, surface 

 glabrous. Cardinal teeth deeply sulcated and crenated ; lateral teeth 

 two in each valve; internal or lower one of the left valve small, but 

 distinct and elevated, and both marked with fine dotted striae. 

 Muscular impressions deep, posterior one rougii. Naker pearly 

 white. 



Remarks. This shell, as will be seen by its dimensions, has a 

 more globose form than perhaps any other Unio. It will stand erect 



