DIATOMACE.E 



-NAVICULEiE. 



307 



formed, from which a new filament begins to sprout for each of 

 the half-frustules ; and when these separate, they carry apart the 

 peduncles which support them, as far as their divergence can take 

 place. It is in this manner that the dichotomous character is 

 given to the entire stipes (Fig. 139). The species of Gromphonema 

 are, with scarcely an exception, inhabitants of fresh water, and are 

 among the commonest forms of Diatomacea?. 



233. Lastly, we come to the large family Naviculece, the mem- 

 bers of which are distinguished by the symmetry of their frustules 

 as well in the lateral as in the front view, and by the presence of 

 a median longitudinal line and central nodule in both valves. In 



Fig. 141. 



Schizonema Grevillii : — A, natural size ; b, portion magnified 

 five diameters ; c, filament magnified 100 diameters ; d, single 

 frustule. 



the Genus Navicula and its allies, the frustules are free or simply 

 adherent to each other ; whilst in another large section they are 

 included within a gelatinous envelope, or are enclosed in a definite 

 tubular or gelatinous frond. Of the genus Navicula an immense 

 number of species have been described, the grounds of separation 



x 2 



