87 



i<-Xt> Valve face not longitudinally undulate; valve face with longi- 

 tudinal folds which appear as heavy costae, folds not conspic- 

 uous in girdle view; girdle view rectangular, naviculoid, 

 wedge-shapedjOr sigmoid; valve view linear, elliptical, ovate, 

 or spirally twisted - SURIRF.T.TA 



wing 



Diagrammatic cross-section 

 of Surirella , (7) 



Fig. 137. Surirella angustata , (5), cells l8-70u long, 6-20u wide; 

 6-9 longitudinal folds in lOu; striae delicate, the 

 only "common" Surirella that is transversely symmetrical, 

 X 1000. 



^<ULI I1IIIHI.I UJLLAi-U-SA*' 



valve view 



girdle view 



Notes 



Fig. 138. Surirella brightwellii , (7), cells 30-50u long, 15-30n 

 wide; longitudinal folds not extended to the center; 

 border raised. X 1000. In Arkansas and Colorado Rivers. 



valve view 



Notes : 



