2i)T 



forated siliceous plates situated between the girdle and each valve. 

 The valves are transversely striated, the striae radiating somewhat 

 in the centre, and there is a straight median raphe with central 

 and polar nodules. 



About four species are known from the freshwaters of the British Islands, 

 of which JA Smith ii Thwaites (length of valves 30 45 /i ; fig. 139) and .I/. 

 Dansei Thwaites are the most generally distributed. 



Family 2. GOMPHONEMACE^J. 



This family of the Naviculoidese is characterized by the wedge- 

 shaped form of the frustules, especially when seen in the girdle-view. 

 In the valve-view they are naviculoid in form, with one pole 

 generally much larger than the other and with sinuate margins. 

 The valves are symmetrical about the sagittal axis (line of raphe) 

 and the raphe is straight and median. Longitudinal septa are 

 present between the valves and the girdle, but they do not extend 

 far into the cavity of the cell. Each frustule possesses a large 

 parietal chromatophore, which is somewhat sinuate and fits closely 

 along one girdle-face, covering the valve-faces and most of the 

 other girdle-face. The frustules are generally attached by their 

 inferior (or smaller) extremities to a branched system of hyaline 

 stalks, which is attached to a sub- 

 stratum. There are only two 

 genera, Gomphonema, in which the 

 frustules are straight, and Rhoico- 

 sphenia, in which they are curved. 

 Two auxospores are formed from 

 two mother-cells without conjuga- 

 tion and grow parallel to each other. 



Genus Gomphonema Ag.,1824. 

 \Gomphonella Rabenh.; Gomphoneis 

 Cleve.] The frustules are of vari- 

 able form in the valve-view, gene- 

 rally with one extremity conspicu- 

 ously larger than the other, often 

 fusiform in outline or sinuate at 

 the margins. Each valve possesses a straight median raphe, and 

 the central and polar nodules are well-marked. The valves are 

 generally strongly striated, the strise more or less radiating from 



B 



Fig. 140. AandB, Gomphonema 

 fjemiiiatum (Lyngb.) Ag., from Caut- 

 'ley Spout, W. Yorks. ( x 400). C, 

 G. cunstrii-tum Ehrenb., from Chip- 

 peuham Fen, Cambridge ( x 400). 



