288 



Bacillariece 



mostly absent, but the polar nodules are disposed near the ventral 

 margin. The girdle-view is generally rectangular. Each frustule 

 possesses two, parietal, plate-like chromatophores. 



Genus Ceratoneis Ehrenb., 1840. The cells are solitary and 

 in the valve-view the ventral or concave side exhibits a more or 

 less prominent central swelling. The apices of the valves are 

 obtuse, subcapitate, or rostrate-capitate. There is a well-marked 

 pseudo-raphe close to the ventral margin and interrupting the 

 transverse striae. The polar and central nodules are comparatively 

 distinct. The girdle-view is linear-rectangular. 



C. Arcus (Ehrenb.) Kiitz. [ = Eunotia Arcus W. Sm.], with obtuse apices, is 

 a frequent Diatom in mountainous areas, particularly in mountain streams ; 

 length of valves 35 60 p. ; fig. 134 D. A variety of it var. Amphioxys 

 (Rabenh.) De Toni distinguished by its produced and subcapitate apices, is 

 also abundant in mountainous districts, and pure gatherings of it are by no 

 means uncommon. The valves reach a length of about 85 p. 



Genus Eunotia Ehrenb., 1837. [Himantidium of various 



authors ; Desmagonium Ehrenb., 

 1848 ; Amphicampa Ehrenb., 

 1849 ; Pseudeunotia Grun., 

 1865 ; Climacidium Ehrenb., 

 1867.] The valve-view is ar- 

 cuate or bow-shaped and the 

 dorsal margin is often undulate 

 or nodulose. The apices are 

 obtuse or more often subcapi- 

 tate. There is no central nodule 

 and the pseudo-raphe is not 

 very evident owing to its posi- 

 tion along the ventral margin. 

 The transverse striae are in- 

 terrupted across the vajve-face. 

 The girdle-view is rectangular. 

 In one section of the genus 

 (Himantidium) the cells are 

 united by their valve-faces to 

 form long, flexuose, ribbon-like 

 filaments. In the other section 



(Eunotia) the cells are generally solitary, but they may be epiphytic 



in clusters on other larger Algae. 



Fig. 134. A, Eunotia pectinalis (Kiitz.) 

 Rabenh. var. undulata Kalfs, from Bail- 

 don, W. Yorks. ( x 600). B, E. robusta 

 Balfs, from Dolgelly, Wales ( x 600). C, 

 E. gracilis (Ehrenb.) Rabenh., from 

 Lerwick, Shetlands ( x 500). D, Cera- 

 toneis Arcus Kiitz., from Cautley Spout, 

 W. Yorks. ( x 520). 



