302 



Bacillariece 



are slightly convex, transversely striated, and furnished with a 

 median keel. 



B. paradoxa Gmel. occurs in the freshwater dykes and drains of the north- 

 east and east of England ; length of valves 60 70 p.. 



Genus Nitzschia Hassall, 1845; em. Grun., 1880. [Tryblionella 

 W. Sm., 1853 ; Grunowia Rabenh., 1864 ; Nitzschiella Rabenh., 

 1864.] The frustules are generally curved, rarely straight, usually 

 free-floating or more rarely forming a thin stratum. Each valve 

 possesses a keel which is displaced to one side, the two keels being 

 displaced to opposite sides of the frustule and therefore diagonally 

 opposite each other. The valve-view is elongated or elliptic- 

 lanceolate, with attenuated and capitate or even rostrate apices, 

 and possesses a row of conspicuous carinal dots at one edge. The 



girdle-view is often elongated with 

 parallel margins and truncate apices, 

 but sometimes the sides are inflated 

 and there is a median constriction. 

 It is occasionally straight, but more 

 often sigmoid. The edge of each 

 keel is furnished with a row of strong 

 carinal dots, which are sometimes pro- 

 longed into short ribs, and the valves 

 are transversely striated. 



This large genus of Diatoms has been 

 divided by Grunow into 22 sections (includ- 

 ing Bacillaria}. There are about 24 British 

 freshwater species, of which N. palea (Kiitz.) 

 W. Sm. (length of valves 2065 /*) and N. 

 sigmoidea (Ehrenb.) W. Sm. (length of valves 



A . E^^Bfl up to 480 /i are the most abundant and at 



the same time exhibit the greatest extremes 

 of size. N. linearis (Ag.) W. Sm. and N. 

 communis Rabenh. are common species, and 

 JV. sinuata (W. Sm.) Grun. is general on 

 dripping rocks. N. acicularis W. Sm. is a 

 small species with greatly produced ex- 

 tremities ; it is often exceedingly abundant 

 in ponds and ditches, and is remarkable for 

 the rapidity of its movements. 



Fig. 143. A and B, two 

 single valves of Nitzschia con- 

 stricta (Kiitz.) Pritch., from 

 Hawksworth, W. Yorks. ( x 500). 

 C and D,N.8i(/moidea (Ehrenb.) 

 W. Sm. ; C, short, straight, ab- 

 normal form in process of di- 

 vision (girdle- view, x 400) ; D, 

 transverse section (after Schiitt, 

 from Pfitzer). 



Genus Hantzschia Grun., 1877. This genus differs principally 

 from Nitzschia in the position of the keels of the two valves, which 



