NAVICTJLA. 429 



stride, and in the frastules being attenuated in the front view 

 towards either extremity. 



3. CYMBELLA ? ARCUS Hass. 

 Plate C. Fig. 6. 



Char. Frustules smooth. In front view contracted towards 

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

 projection in the centre of the concave margin. 



Navicula arcus, in Wiegmann's Archiv. fur Naturg. 1836, 

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



Through this species the genera Cymbella 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 aspect. 



C. Form 0/*frustules various. 

 79. NAVICULA Bory. 



Char. Frustules straight. In front view usually somewhat 

 attenuated towards 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, would appear to be an ex- 

 ceedingly natural one. 



a. Frustules without transverse strice. 



1. NAVICULA PHCGNICENTERON Ehr. 

 Plate CII. Fig. 9. 



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

 attenuated towards either end; in lateral aspect extre- 

 mities lanceolate. 



