NAVICULEA. 53 
Section IT. Vatve NARRow, MORE OR LESS CONVEX. 
It will be observed that the valve, in species typical of 
Section I. (such as Navicula elliptica or N. pretexta), is 
much flattened and broad in proportion to its length, so 
that the entire frustule is very much broader on its S. V. 
than on its M. V., which is linear and comparatively 
narrow. In Section II., however, the relative proportions 
of the valve and of the frustule are entirely reversed 
(especially in the more typical forms, such as N. Northum- 
brica and its allies): the former is highly convex and 
narrow in proportion to its length, while the latter is very 
much broader on its M. V. than on its S. V. But as 
these two opposite types of development are receded from 
in the more aberrant or less typical species, we find the 
distinctive features of the two sections more and more 
diminished until they approximate each other very closely. 
In consequence of this fact an obstacle presents itself 
to the splitting up of the huge genus Navicula into two 
distinct genera. However the two primary sections into 
which I have divided it may be regarded as equivalent to 
subgenera which gradually approximate to each other, or 
become almost merged, through the medium of species in 
which the sectional or subgeneric characters become less 
and less distinct. 
Subsection I. Valve highly convex, compressed laterally into a 
keel elevated towards the extremities and depressed in the 
middle. Frustule on M. V. broader than on S.V., con- 
stricted in the middle and expanded towards the extremities. 
In the peculiar configuration of the valves and of the 
frustule, the species of this subsection are allied to the 
genera Amphiprora and Donkinia, between which and the 
G 
