300 



Bacillariece 



principally differs in the position of the raphe, in the form of the 

 girdle-view, and in the broad striated girdle. 



Most of the species are marine, but A. ovalis Klitz. is a common fresh- 

 water species with valves 50 70 p. in length ; fig. 141 B and C. A very 

 small variety of it var. pediculus Kiitz. occurs as an epiphyte, generally on 

 other Diatoms, such as Nitzschia sigmoidea. 



Genus Epithemia Breb., 1838. The frustules are solitary and 

 free-floating or often epiphytic on other Algae and more highly 



organised plants. In the 

 valve-view they are slightly 

 curved or lunate, with an 

 inner concave margin and 

 an outer convex margin. 

 The valves are apparently 

 without a true raphe, which 

 is replaced by an excentric 

 pseudo-raphe situated close 

 to the concave margin. O. 

 Miiller has shown, however, 

 that one species possesses a 

 true raphe, and it is highly 

 probable that the so-called 

 'pseudo-raphe' of all the 

 other species is morphologic- 

 ally a true raphe. There are 

 strong transverse ribs in the 

 valve-view which represent 

 transverse septa, and the 

 valves are also sculptured 

 with transverse rows of 

 beads or finely punctate 

 striae. The girdle-view is 

 subrectangular, lanceolate, 



or broadly elliptical with truncate, apices. The transverse septa 

 also show as costae in the girdle-view owing to the convex nature 

 of the valves, and they sometimes terminate in small globular 

 expansions at their point of contact with a partial longitudinal 

 septum, which is situated on each side of the girdle between 

 it and the valves. There is either one large chromatophore in 



Fig. 142. A, girdle-view of Epithemia 

 gibba Kiitz., from Wicken Fen, Cambridge. 

 B, valve-view of E. turgida (Ehrenb.) Kiitz., 

 from Keighley, W. Yorks. C, girdle-view 

 of dividing specimen of E. turgida, from 

 Lerwick, Shetlands. D, girdle-view of E. 

 Argus (Ehrenb.) Kiitz., from Wicken Fen, 

 Cambridge. (All x 450.) 





