PLATE XI. 
Fig. 1. Navicuta nosixis, Ehrenberg. S. V. 
2. Navicuta Masor, Kiitzing. a, S.V.; 4, M. Vs 
the latter showing the typical form and relative 
breadth of this surface of the frustule in Sub- 
section II. Section II. of the genus Navicula. 
The M.V. of N. Clepsydra (Pl. X. fig. 26) shows 
the typical form and relative breadth in ‘Sub- 
section I. of the same Section. A comparison 
of the two forms will show the characters on 
which the two subsections are founded. 
