292 Baeittariece 



valve. In some the central nodule forms a wide stauros (Stauro- 

 neis) and in others it is enclosed between two siliceous ridges 

 (Vanheurckia). In a few genera the valves are carinate, or they 

 are unequal, with a straight margin and an inflated one. The 

 valves are striated, the striae being commonly transverse, punctate, 

 and interrupted by the raphe. In Mastogloia the valves are 

 compound, being composed of an ordinary striated valve superposed 

 on a perforated plate. The frustules are mostly solitary and free- 

 floating, but in some forms they are enclosed within a tough mucous 

 envelope which is often much branched ; others are attached to a 

 substratum by hyaline stalks. The chromatophores generally 

 consist of two large parietal plates which are principally disposed 

 over the walls of the girdle-faces. Two auxospores are commonly 

 produced by the conjugation of two mother-cells, which surround 

 themselves with a wide mucous investment. 



There are seven British freshwater genera of the family, distinguished as 

 follows : 



A. Valves simple. 



* Frustules straight and symmetrical in three planes at right angles ; 

 raphe straight. 



t Eaphe straight and simple, with polar and central nodules. 



| Central nodule small Navicula. 



\\ Central nodule forming a stauros Stauroneis. 



tt Raphe straight, enclosed between two longitudinal ribs. 



I Central nodule small Vanheurckia. 



\l Central nodule linear and greatly elon- 

 gated Amphipleura. 



** Frustules sigmoid ; raphe sigmoid Gyrosigma. 



*** Frustules twisted; with sigmoid keel in the 



sagittal line; raphe sigmoid Amphiprora. 



B. Valves composed of two superposed plates Mastogloia. 



Genus Navicula Bory, 1822. [Pinnularia Ehrenb., 1843; 

 Schizonema Ag., 1824 ; Colletouema Breb., 1849 ; Diadesmis Kiitz., 

 1844.] The frustules are solitary and free-floating or enclosed in 

 mucous tubes, rarely united by their flat valve-faces to form ribbon- 

 like filaments. The valves are quite straight and symmetrical 

 with regard to the line of the raphe (sagittal line). The raphe is 

 straight with central and polar nodules. In the girdle-view the 

 frustules are straight and subrectangular. In the valve-view the 

 form of the cell is very variable, although the predominant shape 

 is lanceolate or fusiform. The markings on the valves are more or 



