Tabellariacece 281 



Family 2. Meridionacece. Cells elongate and cuneate, united to 

 form a flat spiral filament or situated on branched gelatinous stalks ; 

 with false septa. Valves with a very indistinct median pseudo-raphe. 



Family 3. Diatomacece. Cells mostly rod-shaped or subrectan- 

 gular, united into band-like or zig-zag filaments; valves with feebly 

 developed transverse septa and a median pseudo-raphe. 



Family 4. Fragilariacece. Cells in girdle-view rectangular, gene- 

 rally united to form ribbon-like filaments. Valves without any false 

 septa and with a median pseudo-raphe. 



Family 5. Eunotiacece. Cells slightly curved or arcuate in the 

 valve-view, solitary or united into band-like filaments. Pseudo-raphe 

 nearer to one edge of the valve. 



Family 1. TABELLARIACE^J. 



The cells are stout and expanded in the girdle-view to form 

 flat, rectangular plates. In the valve-view they are bilaterally 

 symmetrical, linear or linear-elliptic, often with a swollen median 

 portion and sometimes with subcapitate extremities. They are 

 mostly united to form band-like or zig-zag filaments by mucous 

 cushions on the valve-faces or at the angles. In the interior of 

 the frustules are two or more false longitudinal septa. Both valves 

 are precisely similar with a straight median pseudo-raphe (sagittal 

 line). The chromatophores are numerous and granular. 



Genus Tetracyclus Ralfs, 1843; em. Grun., 1862. The 

 frustules are tabular and are united to form short or long, ribbon- 

 like filaments. There are a number of perforated longitudinal 

 septa, which appear in the girdle-view as ribs with thickened 

 apices. There are also false transverse septa at right angles to 

 these, which appear as few or many costa3 (or ribs) in the valve- 

 view. The median portion of the valve may or may not be 

 swollen. 



There are two British species, T. lacustris Ralfs (length of valves up to 

 30 p,; fig. 129 A C), which prefers hilly districts and is often found in the 

 plankton of mountain lakes, and T. rupestris (A. Br.) Grim., a species which 

 occurs on dripping rocks in mountainous areas. 



Genus Tabellaria Ehrenb., 1839. The frustules are tabular 

 and united to form zig-zag filaments, the basal cell often being 

 fixed to the substratum by a mucous cushion at one corner of 

 the valve. There are a number of perforated longitudinal septa, 

 which appear in the girdle-view as prominent lines which fail to 



