NitzscJiiacece 301 



each cell or two smaller ones. Auxospores are formed in pairs 

 from two mother-cells after conjugation. 



The genus is almost entirely freshwater in habit, E. turgida (Ehrenb.) 

 Kiitz. being the most abundant British species; length of valves 70 150/x; 

 fig. 142 B and C. E. gibba Kiitz. (fig. 142 A) and E. zebra (Ehrenb.) Kutz. 

 are also common species in all kinds of localities. E. gibberula Kiitz. var. 

 producta Grun. and E. Argus (Ehreub.) Kiitz. var. alpestris (W. Sm.) Rabenh. 

 often occur in large quantity in mountainous areas. 



Sub-order 4. NITZSCHIOIDE^. 



In this sub -order of the Pennatse the frustules are elongated 

 and asymmetrical, generally with more or less of a sigmoid curva- 

 ture. Each valve possesses a. keel in the sagittal line, and the 

 two keels may be diagonally opposite or both displaced to the 

 same side of the frustule. The edge of the keel is usually furnished 

 with strong carinal dots. In transverse section the frustules are 

 rhombic. 



Family 1. NITZSCHIACE^. 



This is the only family of the sub-order. The valve-view is 

 generally straight, with attenuated apices, and a row of carinal 

 dots, either median or at one edge. The girdle-view is linear or 

 sigmoid, with truncate apices. The keel is along the sagittal line 

 of the valve and contains a true raphe. The chromatophores are 

 variable; the cells may contain one diagonal plate-like chro- 

 matophore, or two smaller diagonal plates, or a large parietal plate 

 the median portion of which is opposite one girdle-face. 



There are three genera, distinguished as follows : 



A. Keel median ; valves a little convex ; cells joined 



to form loose bands Bacillaria. 



B. Keel displaced to one side ; valves convex ; cells free. 



* Keels of two valves diagonally opposite Nitzschia. 



** Keels of two valves displaced to same side of 



frustule Hantzschia. 



Genus Bacillaria Gmelin, 1788. The frustules are straight 

 and united to form plate-like or ribbon-like colonies, which exhibit 

 a gliding movement of one frustule over another. The girdle-view 

 is linear, with truncate apices and with a row of carinal dots along 

 each lateral margin. The valve-view is narrowly linear, with 

 attenuated apices and a median row of carinal dots. The valves 



