380 KANSAS UNIVERSITY SCIENCE BULLETIN. 



in all the Kansas river systems, but is most plentiful in the 

 Marais des Cygnes. It is an inhabitant of muddy river-beds. 



The variation is considerable. The elongation of the pos- 

 terior slope, the excavation of the radial furrow and the 

 inflation all exhibit it to a marked degree. The young speci- 

 mens generally bear a series of green rays over the umbones, 

 but this character is not often present in the adult shell. 



The two species with which mbiginosa might be confused 

 are (}. trigona and Q. coccinea. Under the descriptions of these 

 two species will be found a statement of the main distinguish- 

 ing characters. 



Quadrula trigona Lea. Not figured. 



Unio trigonus Lea, Trans. Amer. Phil. Soc, iv, 1831, p. 110. pi. XVI, 

 fig. 40. 



"Shell trigonal, thick and heavy, much inflated, broadly 

 rounded, and behind the posterior margin being produced 

 vent rally ; dorsal margin curved ; ventral margin more or 

 less sinuous ; surface more or less shining, roughened by 

 lines of growth ; umbones large, elevated, inflated, dark 

 brown, directed anteriorly, aud unmarked except by growth 

 lines ; anterior umboidal slope short and flatly rounded ; 

 posterior slope strongly angled, with an excavation on each 

 side of the angle, which reaches from the apex to the ven- 

 tral border, where it forms a decided beak-like projection ; 

 viewed anteriorly, the shell is strongly heart-shaped, resem- 

 bling in this respect some cardia, particularly Isocardia cor. 

 Ligament short, wide, stout, very dark brown or black ; epi- 

 dermis reddish or blackish horn, unmarked by rays; cardi- 

 nal teeth double in the left and single in the right valve, 

 very stout, generally not much elevated, triangular, diverg- 

 ing, very deeply grooved and striated ; lateral teeth short, 

 solid, elevated, lamellar serrated, directed and curved ven- 

 trally ; the right lateral aud the lower left lateral have each 

 a depression and a rudiment of an additional tooth ; connect- 

 ing bridge thick, wide, flat, smooth ; anterior adductor mus- 

 cle scar forming a truncated oval, very deeply excavated, 

 striated ; posterior adductor muscle scar oval, well impressed, 

 striated ; protractor pedis muscle scar wider than long, deeply 



