4 NAVICULEA. 
vary much in size in different species, from being so 
minute (in N. rhomboides and others) as to be only visible 
with the highest magnifying-powers, to being large and 
distinctly visible with a low power (as in N. pretexta, N. 
granulata, &c.). The degree of segregation and of aggre- 
gation of the granules forming the striz is also, in different 
species, subject to great variation. In N. granulata and 
others they are widely separated by a distinct interval be- 
tween them; in N. nobilis and its allies, on the contrary, 
they are so closely packed as to present the appearance of 
smooth ridges or costz on the surface of the valve. Be- 
tween these opposite types every possible intermediate 
degree of variation prevails in different species, so that no 
definite limit can be drawn to separate the species with 
visibly granular striz from those whose striz are apparently 
costate. Consequently the genus Pinnularia of Ehren- 
berg, as reconstructed by the late Professor Smith*, must 
necessarily be abolished, not having been founded on any 
structural peculiarity which is more than purely specific. 
Undoubtedly the construction of this genus, and its adop- 
tion by several observers, has had the pernicious effect of 
separating species closely allied, and placing them in 
different genera, thus creating much confusion in the 
nomenclature of the subject. In numerous instances both 
Ehrenberg and Smith disregarded the distinction they had 
drawn between Navicula and Pinnularia in the classifi- 
cation of certain species. In the present work the latter 
genus has been reduced to a synonym of the former. In 
this respect the example of Prof. Kiitzing, M. De Brébisson, 
aud Mr. Ralfs has been followed. 
* Synop. of the British Diatomacese, vol. i. p. 54. 
