Naviculacece 



293 



less transverse, sometimes somewhat radiating, and are arranged 

 in relation to the line of the raphe. They consist of striae or 

 costae of variable strength and are frequently composed of rows of 

 punctae. The striae never quite reach up to the raphe, and some- 

 times there is a hyaline area of considerable size on each side of 

 the central nodule. In rare cases the striae are interrupted by a 

 smooth longitudinal area on each side of the raphe and parallel to 

 it. The chromatophores consist of two large parietal plates and in 

 the formation of auxospores two spores are formed by the con- 

 jugation of two mother-cells. 



Navicula is much the largest genus of Diatoms, or indeed, of 

 any group of Algae, embracing upwards of 1,000 species, which 



Fig. 136. A, Navicula alpina (W. Sm.) Ralfs, from Lerwick, Shetlands (x400). 

 B, X. iiridis Kiitz., from Baildon, W. Yorks. (x400). C, JV. sphcerophora 

 Kiitz.,from Wimpole Park, Cambridge ( x 500). D, N. serians (Breb.) Kiitz. , 

 from Mickle Fell, N. Yorks. ( x 400). E, Stauroneis Pluxnicenteron (Nitzsch) 

 Ehreub., from Adel Bog, W. Yorks. ( x 400). F, St. acuta W. Sm., from 

 Shipley Glen, W. Yorks. ( x 450). 



occur widely distributed in fresh, brackish and salt water. Many 

 are also known in a fossil condition. It has been subdivided by 

 Cleve into the three subgenera Navicula, Stauroneis and Dictyoneis. 

 The first of these subgenera is divided into the two groups 

 Eunavicula and Schizonema, and there are twenty-two sections of 

 the first group, twelve of which contain British freshwater species ; 

 in the second group there are two sections, one of which contains 



