299 



Genus Cocconema Ehrenb., 1829. [Cymbella Ag., 1830; 

 Encyonema Kiitz., 1833.] The frustules are asymmetrical, straight, 

 or more often sublunate,being strongly 

 attenuated from the middle towards 

 the extremities, which are obtuse. In 

 the valve-view one margin is strongly 

 convex, whereas the other may be 

 slightly convex or concave. In the 

 latter case it is generally tumid in 

 the middle. The raphe is well-marked 

 and is nearer the concave or the less 

 convex side of the valve. The valves 

 are striated, the striae slightly radiat- 

 ing from the raphe and consisting in 

 many species of distinct series of dots. 

 The girdle- view is straight, often sub- 

 rectangular, and the actual girdle is 

 generally very narrow. In some species 

 the frustules are free, in others they 

 are stipitate, and in others they are 

 enclosed in gelatinous tubes, these 

 differences in habit having been form- 

 erly utilized as generic characters. 



C. lanceolatum Ehrenb. is the largest 

 and one of the most abundant species of 

 the genus; length of valves 80 150 p 

 fig. 141 A. C. Cistula Ehrenb. and C. cymbiforme Ehrenb. are smaller species 

 almost equally abundant. G. Ehrenbergii (Kiitz.) nob. is a large species of 

 rare occurrence. C. cuspidatum (Kiitz.) nob. is widely distributed and often 

 abundant on wet rocks in mountainous regions, frequently forming gelatinous 

 masses of a greyish-brown colour. The frustules of C. prostratum (Berk.) 

 nob., C. ccespitosum (Kiitz.) nob. and C. gracile (Rabenh.) nob. occur in more 

 or less linear series enclosed in gelatinous tubes. 



Genus Amphora Ehrenb., 1831. The frustules are asym- 

 metrical, curved or sublunate in the valve- view, with a well-marked 

 raphe near the concave side. The central nodule is adjacent to 

 the concave margin and is sometimes widened into a stauros. The 

 striation of the valves is similar to that of Cocconema. The 

 girdle-view is elliptical with truncate apices and the girdle is 

 broad with irregular longitudinal striations. The chromatophore 

 is similar to that of Cocconema, from which genus Amphora 



Fig. 141. A, Cocconema lan- 

 ceolatum Ehrenb., from Shipley 

 Glen, W. Yorks. ( x 500). B and 

 C, Amphora ovalis Kiitz., from 

 Moidart, Inverness (x500). 



