Naviculacece 291 



There are many marine species of this genus, but only two C. Pediculus 

 Ehrenb. and C. Placentula Ehrenb. (fig. 135 D F) inhabit fresh water, both 

 of them being abundant and widely distributed all over the British Islands. 

 C. Pediculus is distinguished from C. Placentula by the form of the valves, 

 which are more attenuated towards the poles, and by the different markings. 

 They are both of approximately the same size (12 35 p in length) and occur 

 as epiphytes on filamentous Algse and other water-plants. Filaments of 

 Cladophora and Vaucheria are often completely covered with these epiphytes, 

 which are attached by their flat valve-faces. 



Sub-order 3. NAVICULOIDE^E. 



This is much the largest sub-order of the Pennatae and is 

 characterized by the presence of a true raphe on each valve of the 

 frustule, which is furnished with a central and two polar nodules. 

 The valves are usually without a keel, but if present it is situated 

 in the line of the raphe (sagittal line). The frustules may be 

 free-floating, occurring in large numbers in mucous tubes, or 

 attached to some fixed object by hyaline stalks. There are three 

 well-marked families. 



Family 1. Naviculacece. Valves straight, rarely oblique or sig- 

 moid, generally with a perfectly straight raphe, more rarely with an 

 oblique one; mostly symmetrical about a longitudinal axis (line of 

 raphe) and a transverse axis; girdle-view generally symmetrical and 

 rectangular. 



Family 2. Gomphonemacece. Valves mostly straight (rarely curved) 

 and symmetrical about a longitudinal axis (line of raphe) : in both 

 valve- and girdle-view wedge-shaped. 



Family 3. Cocconemacece. Valves always curved, symmetrical 

 about one longitudinal plane and a median transverse plane ; valve- 

 view generally curved, often sublunate ; girdle-view straight and sub- 

 rectangular. 



Family 1. NAVICULACEJE. 



The Diatoms of this family are exceedingly numerous and 

 well-defined. The frustules are generally straight and symmetrical 

 in three planes at right angles to each other (Navicula, Vanheurckia, 

 etc.), or they may be sigmoid (Gyrosigma), or twisted (Rhoicosigma). 

 The valves are almost always elongated, although in a few genera 

 (Amphiprora) they may be relatively short. The raphe is somewhat 

 variable ; it is generally median with central and polar nodules, 

 but it may take up a curved position according to the form of the 



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