286 



Bacillariece 



fine or rather coarse and composed of bead-like punctulations. 

 The girdle-view is rectangular. The frustules are joined together 

 by their valve-faces to form ribbon-like filaments, or they are 

 united by small mucous cushions at their corners to form zig-zag 

 filaments. In the section Eufrogilaria Ralfs the pseudo-raphe is 

 very insignificant and the chromatophores are small and granular ; 

 in the section Staurosira Ehrenb. (= Odontidium Klitz.) the 

 pseudo-raphe is broad, often lanceolate, and the chromatophores 

 are plate-like as in Synedra. 



F. capudna Desmaz. (length of valves 30 60 p.; fig. 132 C and D) is 

 much the commonest freshwater species, but F. mutabilis (W. Sm.) Grun. 

 and F. virescens Ralfs are also general. F. construens (Ehreub.) Grun. and 

 F. Crotonensis (A. M. Edw.) Kitton are rarer freshwater species. 



Genus Synedra Ehrenb., 1831. The frustules are much 

 elongated and occasionally bent or somewhat undulated. The 



valve-view is commonly linear or 

 linear-lanceolate, with obtuse or 

 subcapitate extremities, and there 

 is generally a pseudo-raphe or a 

 hyaline sagittal line. In the centre 

 of the valve-face there is generally 

 a small rounded hyaline space, and 

 sometimes central and polar nodules 

 are present. The valves are finely 

 striated, the striae being transverse 

 and reaching up to the pseudo- 

 raphe on each side. The girdle- 

 view is elongated with truncate 

 apices. Each frustule contains two 

 plate-like chromatophores with un- 

 dulated or indented edges. Most 

 of the species are solitary, but 

 some occur clustered in radiating 

 or fan-shaped colonies. 



Fig. 132. A and B, Synedra 

 pulchella Kiitz., from Cambridge 

 (x500). C and D, Fragilaria 

 capudna Desmaz., from Shipley, 

 W. Yorks. (x520). 



There are about eight British fresh- 

 water species, several of which are 

 common, being found in almost every 



kind of suitable locality and often occurring in immense abundance in the 

 waters of lakes and springs. S. Ulna (Nitzsch) Ehrenb. (length of valves 

 150 250 ji), S. pulchella Kiitz. (length 60 130 /x ; fig. 132 A and B) and 



