332 KANSAS UNIVERSITY SCIENCE BULLETIN. 



S. riigosa is distributed throughout the Mississippi valley, 

 and is also found in Manitoba, in the St. Lawrence drainage, 

 and at Columbus, Miss. In Kansas it has been found only 

 in the Neosho river and two of its tributaries, the Cottonwood 

 river and Fall river. I do not believe this to be a common 

 species at any place in the state. 



There will be no difficulty in identifying this form. The 

 peculiar lateral teeth rudiments will at once indicate the 

 genus and the rugose posterior slope the species. The two 

 sexes are easily distinguished ; the female is much shorter 

 and more inflated than the male. Aside from the sexual va- 

 riation, the only other decided variant is the number and 

 character of the posterior undulations. 



Symphynota complanata Barnes. Plate LXXVI, fig. 2. 



Alasmodonta complanata Barnes, Amer. Jour, of Sci. and Arts, vi, 

 1823, p. 278, pi. xin, fig. 21. 



Shell large, moderately thick anteriorly, thinner poste- 

 riorly, wide ellipsoid, compressed in male, less so in fe- 

 male ; young specimens strongly alate, but this character 

 often lost in older specimens. Anterior margin rounded ; ven- 

 tral margin well rounded in female, less so in the male ; pos- 

 terior margin roundly biangulate in young, rounded in old 

 specimens. Wing high in old, pointed in young specimens. 

 Umboidal ratio, 0.20 to 0.25. Umbones flattened and marked 

 with four or five rows of double-looped, coarse, v -shaped 

 ridges. Umboidal slopes flattened in male, flatly rounded in 

 female. Epidermis in young specimens smooth, shiny, light 

 to dark brown, sometimes rayed, in old specimens rough, dark 

 brown to black, eradiate. Lines of growth raised, continuous, 

 numerous, sometimes slightly wavy at the base of the wing. 

 Ligament long, light brown. 



Interior : Pseudocardinals variable, generally double in left 

 and single in the right valve, large, pyramidally serrate. 

 Lateral teeth indicated by a low, irregular ridge on each 

 hinge plate. Interdentum sometimes present, sometimes ex- 



