SCAMMON : THE UNIONIDiE OP KANSAS, PART I. 347 



small and weak. Laterals short, stout, curved slightly ven- 

 trally. Interdentum broad and flat, but cut away in the 

 right valve to receive the posterior pseudocardinal. Anterior 

 muscle cicatrices both long and narrow, of moderate depth, 

 and with roughened floors ; the anterior pseudocardinal often 

 curved about half-way around the base of the pseudocardi- 

 nals. Posterior scars large, lightly impressed, longer than 

 wide, confluent. Pallial line crenulate and heavily impressed 

 anteriorly. Dorsal scars well impressed, scattered over the 

 ventral surface of the interdentum. Cavity of the shell quite 

 deep, of the beaks large. Nacre pure white, very iridescent 

 along the posterior margin. 



Q. heros is found throughout the Mississippi valley, east 

 into Alabama, west into Mexico. It has been reported from 

 all the drainage systems in Kansas. In the Kansas and the 

 southern drainage basins it is a rare species. In the Marais 

 des Cygnes system it is occasionally found, but it is not at all 

 common. Although this is said to be the largest Unio of 

 American waters, I have not seen specimens of any great 

 size from the Kansas streams. There will be no difficulty in 

 distinguishing heros from the other undulate Unios. It is the 

 only form in which the umbones and the greater part of the 

 anterior umboidal slope are covered with pustules. 



Quadrula cylindrica Say. Plate LXXXI, fig. 2. 



Unio cylindricus Say, Nich. Encyc, II, 1816, pi. IV, fig. 3. 



Shell of moderate size, elongate rectangular, thick, inflated, 

 nodulated, height about one-third of the length. Anterior 

 margin almost semicircular ; ventral margin straight except 

 where it is inbowed by a long, very shallow indentation ; 

 posterior margin bluntly pointed, the ventro-posterior margin 

 short and straight, dorsal posterior margin longer and in- 

 curved ; dorsal margin straight and meeting the dorsal pos- 

 terior margin at a fairly constant angle. Umboidal ratio, ap- 



