Diatoms 



III 



of water moss or a drop of sediment from the bottom 

 without finding Naviculas in the mount. They are 

 more abundant shoreward than in the open waters of 

 the lake. The ''white-cross diatom" Stauroneis (d), is a 

 kindred form, easily recognizable by the smooth cross- 

 band which replaces the middle nodule of Navicula. 



Fig. 35. Diatoms. 



a, valve view showing middle and end nodules, and b, girdle view of Navicula. c, another 

 species of Navicula; d, Stauroneis; e, valve view and /, girdle view of Tahellaria; g, 

 Synedra; h, Gyrosigma; 1, a gelatinous cord-like cluster of Encyonema showing girdle 

 view of nine individuals and valve view of three. .;', valve view and k, girdle view of 

 Melosira; I, Stephanodiscus; m, ^l/erid/ow colony, with a single detached individual shown 

 in valve view below; n, a small colony of Asterionella; o, valve view, and p, girdle view 

 of Camplylodiscus; q, cluster of Cocconema. (Figures mostly after WoUe). 



Tahellaria (e and /) is a thin flat-celled diatom that 

 forms ribbon-like bands, the cells being apposed, valve 

 to valve. Often the ribbons are broken into rectangu- 

 lar blocks of cells which hang together in zig-zag lines 

 by the corners of the rectangles. The single cell is long- 

 rectangular in girdle view (slightly swollen in the middle 

 and at each end, as shown at e, in valve view), and is 

 traversed by two or more intermediate septa. Tahel- 

 laria abounds in the cool waters of our deeper northern 

 lakes, at all seasons of the year. It is much less common 

 in streams. 



