128 



FRESH-WATER BIOLOGY 



20(23,24,25) Central nodule small, rounded, or slightly elongated. ... 21 

 21(22) No lateral longitudinal areas of transverse septa. . . NavictUa Bory. 



A form which grows in gelatinous tubes is regarded by some 

 authors as a dififerent genus Schizonema but others regard it as 

 a true Navicula. 



Fig. 77. Navicula rhynchocephala Kiitzing. X 557- (Original.) 



22 (21) Two lateral longitudinal areas of transverse septa. Mostly imbedded 



in a gelatinous pseudothaUus. . . . Mastogloia Thwaites. 



In shape, Mastogloia resembles Navictda, but is distinguished from it by 

 the gelatinous envelope and the presence of lateral, transverse, siliceous septa 

 or plates which divide the lateral regions of the cells into small compart- 

 ments. There are transverse striations on the valves. It is not a very 

 common genus in America. 



Fig. 78. Mastogloia smithii Thwaites. X about 300. (After Smith.) 

 A B 



23 (20, 24, 25) Central nodule broad, extending to near the margin of the valves. 



Stauroneis Ehrenberg. 



Siauroneis occurs frequently in all 



bodies of water and is a constituent of 



the diatomaceous flora which forms large 



siliceous deposits at the bottom of lakes. 



Fig. 79. Siauroneis anceps Ehrenberg. 



X 600. fOriginal.) 



24 (20, 23, 25) Central nodule elongated to a short rod. Borne on gelatinous 



stalks Brebissonia Grun. 





Fig. 80. Brebissonia 

 sp. X 580. (Original.) 



25 (20, 23, 24) Central and end nodules elongated, enclosed with the raphe 

 between two longitudinal, parallel, sihceous ribs. Frus- 

 tules sometimes borne in gelatinous tubes. 



Vanheurchia Brebisson. 



Fig. 81. 



Vanlteurckia rhomhoides Ehrenberg. 

 (After West.) 



X370. 



26 (13) Valves asymmetrical with reference to the raphe or to a longitudinal 



axis; raphe arched, or nearer one margin than the other. 



Family Cymbellaceae .' . 27 



27 (28) Valves greatly convex; girdle side elliptical or oval. 



Amphora Ehrenberg. 



^ Van Heurck regards A mphora as one of the most difficult 

 jjjjjjg genera of diatoms and notes that over 200 species have 



~ been placed in this genus. It is believed that it origi- 

 nated from Cymbella. 



— ,--T-~*>.-^T>r~~^^rt- i^ ^'°" 8^' Ampiiora ovalis KUtzing. a. Valve side; 6. girdle 



rgj JjMljjiijJ I^^i^^ side. X6oo. (Original.) 



a8 (27) Valves flat or only slightly convex 29 



