The Nautilus. 



Vol. XXIX. APRIL, 1916. No. 12 



A NEW GENUS AND SPECIES OF NAIAD FROM THE JAMES BIVEB AT 

 HUEON, SOUTH DAKOTA. 



RY WILLIAM B. MARSHALL. 



CoKERiA, new genus. 



Characters those of the new species described below. 

 CoKERiA souTHALLi, ncw species. Plate IV. 



Shell gaping anteriorly, moderately thin, subquadrangular 

 tending to subtriangular ; abruptly rounded anteriorly, pos- 

 terior margin nearly straight and perpendicular, dorsal margin 

 straight, ventral margin gently curving throughout its entire 

 length. Valves widest just in front of the posterior ventral 

 angle. Umbones at about the anterior third of the dorsal mar- 

 gin, high and incurved, the second growth line nearly horizontal. 

 Posterior dorsal ridge very high, obtusely angular. Posterior 

 area descending abruptly from the ridge and somewhat wing-like. 

 Greatest inflation is along the umbonal ridge. A pronounced 

 furrow extends from the umbo to the lower third of the posterior 

 margin. Seven rest- periods distinctly marked by concentric 

 dark lines. Anteriorly from the middle portion a bold rounded 

 rib occurs just below each rest-period. Dorsal area faintly 

 radiately striate. Color, varying shades of chestnut, lighter 

 (nearly straw color) anteriorly, darker posteriorly, rest stages 

 sharply defined by blackish lines. Umbonal region faintly 



