320 KANSAS UNIVERSITY SCIENCE BULLETIN. 



of very fine undulating ridges. Umboidal slopes flattened 

 dorsally but decidedly curved marginally. Posterior um- 

 boidal ridge prominent but rounded. Epidermis light horn 

 color (often with an olive-green cast) to dark chestnut-brown. 

 Rays either fine, dark, and wavy, or broad and interrupted. 

 Lines of growth numerous, coarse, and often imbricated. 

 Lunule large and elongate. Ligament short and stout. 



Interior : Pseudocardinals small, low, serrate, bluntly pyra- 

 midal, single in the right and double in the left valve. Lat- 

 erals short, heavy, slightly curved, oblique. Interdentum 

 quite long, smooth, broad. Anterior adductor cicatrix deeply 

 pitted, elongate, placed in front of the pseudocardinals. Pos- 

 terior scars deeply impresssd and distinct, the retractor al- 

 most hidden, and placed in on the tip of the lateral tooth ; 

 adductor scar elongate and pointed posteriorly. Dorsal mus- 

 cle scar large and well impressed on the lower surface of the 

 interdentum. Pallial line impressed its entire length. Bran- 

 chial area well impressed, cavity of shell small. Beaks prac- 

 tically without a cavity. Nacre milk-white, slightly iridescent 

 posteriorly. 



The range of this species is throughout the southern portion 

 of the Mississippi valley, the Ohio river drainage, and the pe- 

 ninsula of Michigan. In Kansas it is found quite commonly 

 in the clear-water rivers of the southern drainage, particularly 

 in the smaller streams. One specimen has been reported by 

 Call from the Wakarusa river, but careful collecting along 

 that stream at intervals during the last five years has failed 

 to bring to light any more specimens. In the southern rivers 

 of the state the shell reaches unusual size, some specimens 

 in the State Agricultural College, collected by Professor 

 Popenoe, have a length of over I L0 nun. 



This form varies much in shape, and often presents a 

 humped appearance, which has led to its description as a 

 separate species. There are, however, all gradations between 

 this form and the typical one. This species may be confused 



