166 Kansas Academy of Science. 



Tribe I. Fragilarioideae. 



Key to the genera of FragUarioidew . 



I. Frustules elongated, straight or slightly curved, united at base in 

 fan-shaped clusters; usually straight with almost parallel sides, 

 transversely striated and mostly with pseudoraphe Synedra. 



II. Frustules attached side by side into a ribbon; valves elongated with 

 lanceolate ends; frustules in girdle view rectangular and in contact 

 with each other entire length Fragilaria, 



99. Synedra Ehrbg. Plate IV, fig. 38. 



Kansas: Widely distributed, found over twenty times. 

 Nebraska (2-3-24-25); Iowa (4). 



100. Fragilaria Lyngby. Plate IV, fig. 33. 



Kansas: Douglass, August, 1915; Independence, April, 



1916; Horton, June, 1916; Paola, July, 1916. 

 Nebraska (1-2-24-25); Iowa (4). 



Tribe II. Aclinanthoideie. 



Cocconeis, the only genus of the Achnanthoideae thus far identified; 

 frustules of the valves unlike; valves broadly oval. 



101. Cocconeis Ehrbg. Plate IV, fig 37. 



Kansas: This genus was found at Cedarvale in Au- 

 gust, 1916. 

 Nebraska (2-24-25); Iowa (4). 



Tribe III. Naviculoideas. 



Key to the genera of Navicidoidea'. 



I. Frustules symmetrical, valves boat-shaped; marked with trans- 

 verse furrows having a slight radial tendency; some long and 

 narrow and some short and wide; ends either pointed or rounded. 



Navicula. 



II. Very similar to Navicula, frustules rectangular in girdle view. 

 A narrow clear space along the raphe and a wider transverse clear 

 space at the medial nodule extending to the sides of the valve gives 

 the valves a marking having the appearance of a cross. 



Stauroneis. 



III. Frustules much the same as Navicula, but with axis turned like 

 a letter S. Raphe sigmoidal Pleurosigma. 



IV. Frustules borne on pedicels more or less branched. Valves bilat- 

 erally symmetrical, wedge shaped with central nodule near the 

 large end Gomphonema. 



V. Frustules elongated symmetrical with respect to the minor axis. 

 Valves with one side very convex and the other side hardly at all 

 convex, surface transversely striated CymheUa. 



VI. Frustules, when young, enclosed in a mucilaginous tube, in which 

 they multiply by division; valves more or less cymbiform; a 

 straight raphe divides the valves into two unequal parts. 



Encyconema. 



VII. Frustules ovoidal to rectangular in girdle view. Valves extremely 



unsymmetrical with a convex and concave side, an eccentric raphe, 



etc. The transverse striae radiate somewhat from the median 



nodule Amphora. 



