SvN'orsis Fresh-Water Moi,r,uscA 



83 



mp 



Fig. 222 

 Type: Unio personatus Say, fig. 221. Animal, T. rangiana (Lea), fig. 222. 



Genus incertcc sedis. 



Genus COKERIA Marshall, 1916. 



'•'Shell gaping anteriorly, moderately thin, subquadrangular to subtrian- 

 gular; abruptly rounded anteriorly, posterior 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 maro-in 



Fig. 223 



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

 dorsal ridge very high, obtusely angular. Posterior area descending abrupt- 

 ly from the ridge and somewhat Aving-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 con- 



