282 



Bacillariece 





reach the middle of the cell. Sometimes instead of a large 

 perforation in a septum, the septa are alternate, and only extend 

 about two-thirds the distance from one end of the valve to the 

 centre. The valve-view is sublinear, more or less strongly swollen 

 in the middle and subcapitate at the extremities; its .surface is 

 transversely striated, the striations being slightly broken in the 

 middle, thus indicating the presence of a rudimentary pseudo- 

 raphe. The auxospores are formed two in each mother-cell. 



Fig. 129. A C, Tetracyclus lacustris Ralfs, from the plankton of Loch Shin, 

 Sutherland. D and E, Tabellaritt fenestrata (Lyngb.) Kiitz., from Mickle Fell, 

 N. Yorks. F and G, T. flocculosd (Roth) Kiitz., from Mickle Fell, N. Yorks. 

 (All x500.) 



There are two common British species, of which T . flocculosa (Roth) Kiitz. 

 (tig. 129 F and G) is the most abundant. T fenestrata (Lyngb.) Kiitz. (fig. 

 129 D and E) has more slender and elongate valves, the largest specimens 

 noticed having reached a length of 137 /u. 1 . A pretty variety of the latter 

 species var. asterionelloides Grun. in which the frustules are arranged in 

 circles, is known from the plankton of Loughs Neagh and Beg in Ireland, and 

 from that of many lochs in Scotland and in the English Lake District. It 

 is probably general in the British freshwater plankton and there are two very 

 distinct forms of it. 



Genus Diatomella Grev., 1855. The frustules are tabular, 

 solitary, or joined to form ribbon-like filaments. There are only 



1 West & G. S. West in Trans. Roy. Irish Acad. 1902, vol. xxxii, sect. B, part 1, 

 p. 90, t. iii, f. 8. 



