284 KANSAS UNIVERSITY SCIENCE BULLETIN. 



Shell small, fairly solid, roundly elongate, triangular in 

 outline, inflated, females particularly so. Anterior maring 

 projecting and decidedly rounded ; ventral margin gently 

 and regularly bowed, posterior margin oblique, straight or 

 slightly rounded ; dorsal margin short, straight or slightly 

 curved. Umbones full, high, decurved. Anterior and lat- 

 eral slopes fully rounded, umboidal ridge well marked ; pos- 

 terior slope abrupt and straight or slightly excavated, marked 

 with numerous fine, continuous, very slightly elevated ridges. 

 Epidermis smooth and shining in most cases, straw-yellow 

 to honey color, strongly marked with rays of dark green 

 made up of arrow-shaped spots. Lines of growth smooth, 

 distinct, and continuous. Ligament short, thick, light or 

 dark brown, lunule rather large. 



Interior: Pseudocardinals high, erect, pointed, slightly 

 serrate at the apices, double in each valve. Interdentum 

 narrow and rounded, cut away in the right valve. Laterals 

 short, slightly curved, high, thick. Anterior adductor cica- 

 trix much longer than wide, well excavated. Posterior scars 

 large, well impressed, distinct. Dorsal muscle scars large, 

 three to five in number, located on the lower surface of the 

 interdentum. Pallial line impressed in its anterior two- 

 thirds. Cavity of the beaks and of the shell deep. Nacre 

 milky white. 



Height. Length, Breadth. Um. ra. 



32* 0.50 27 0.30 c? 



T. triquetra is distributed over the Mississippi region from 

 western New York to Indian Territory. Its only recorded 

 occurrence in Kansas is in the Marais des Cygnes river at Ot- 

 tawa. It will probably be found throughout the eastern half 

 of the state, although it is without doubt a rare species. 



This is a distinctive species which will hardly be confused 

 with any other found in the state. In no other form is the 

 sexual dimorphism more pronounced ; the female shell has 

 an inflation which appears almost abnormal. 



♦All measurements are given in millimeters. 



