Mr. C. Mereschkowsky on Stauronella. 427 
occupying the right and left side of the frustule (see diagram 
fig. 1), in the Nitzschies each plate occupies the superior and 
inferior half of the frustule, thus being separated not by a 
longitudinal but by a transverse interval along the short axis 
of the frustule (fig. 3 of the diagram). ‘This arrangement may 
be termed transverse disposition, as opposed to the longitudinal 
disposition of the Naviculaceee. Whenever there are two 
chromatophore-plates in the Naviculacez they may be distin- 
guished, according to their position, as the right and the left, 
Diagram showing the typical disposition of the two chromatophore- 
plates in Naviculacee (figs. 1,2) and Nitzschiee (figs. 3, 4). 
Figs. 1 and 8, valve-view; figs. 2 and 4, transverse sections. 
v, V, valves ; z, 2, connecting-zones. 
while in the Nitzschiee when there are two plates they 
may be distinguished not as the right and left, but as the 
superior and inferior. Even in such cases as Cylindrotheca, 
Niteschia dissipata, Homeocladium Martianum, &c., where 
the number of chromatophores is more than two, they form 
two groups—a superior and an inferior—separated by a 
transverse interval, instead of forming a right and left group, 
as is the case in Okedenia, where the number of chromato- 
phores is also large. 
I can positively affirm that I have not met a single case of 
