NAVICULA. 429 



stria3j and in the f'rustules being attenuated in tlie front A'icw 

 towards either extr£mity. 



3. CyMBELLA ? ARCUS Hass. 



Plate C. Fig. 6. 



Char. Frustules smooth. In front view contracted toioards 

 either end; in lateral aspect curved, attenuated, loith a 

 projection in the centre of the concave margin. 

 Navicula arcns, in Wicgmann's Archiv. fur Naturg. 183G, 

 p. 243, 244. ; Die Infus. p. 182. t. xxi. fig. 10. 



Through this species the genera Cymhella and Navicula 

 would appear to be united, it agreeing with the first in the 

 semilunar and curved form of the frustule in its lateral view, 

 and with the latter in its attenuation in its anterior asiiect. 



C. Form o/* frustules various. 

 79. NAVICULA ^ory. 



Char. Frustules straight. In front view usually someivhat 

 attenuated toioards either end, extremities generally trun- 

 cate ; in lateral aspect boat-shaped, extremities often 

 acute. 



Derivation. From navicula, a little boat. 



This genus, as limited to those species, which the above 

 definition is intended to include, Avould appear to be an ex- 

 ceedingly natural one. 



a. Frustules ivithout transverse strice. 



1. Navicula ph(enicenteron Ehr. 

 Plate CII. Fig. 9. 



Char. Frustules elongated, smooth, large. In front view 

 attenuated toicards either end; in lateral aspect extre- 

 mities lanceolate. 



