399 
tion of outline. I have in my possession a slide of it, from 
the collection of the late Professor Walker Arnott, with the 
frustulcs almost linear, not at all inflated in the middle, nor 
at the extremities, but tapering slightly from the middle to 
the gently rounded extremities. This is the E. parallela of 
Grunow ; it is a mere variety, however, and not a distinct 
species. 
Rabenhorst (loc. cit.) describes E. (jibba as a fresli-water 
and marine (!) species, and E. ventricosa as fresh-water and 
sub-marine. But actual observation has convinced me that 
it is strictly a fresh-water form, and parasitic on aquatic 
plants in clear still water. That its frustules should be 
found in the mud of tidal estuaries, mixed with marine and 
brackish water species, may reasonably be expected ; but it 
is never found living, so far as my observation goes, in salt 
water. Indeed, long and ample experience has satisfied me 
that fresh-water diatomes are never propagated in water 
impregnated with salt, as are the tidal estuaries of our rivers. 
The converse holds good with brackish water and marine 
species, which are never found in springs, lakes, ponds, or 
streams completely removed from tidal impregnation. 
The jelly-looking flakes floating on the surface of the 
same pond at Wallington were also exceedingly rich in 
Diatomacese, being composed of Pleurosigma attenucitum. 
Sin. ; Cymbella Ehrenbergii, Kiitz. ; Amphora oralis, Ktz. ; 
Pinnularia Dactylus, Ehr. ; Campylodiscus noricus, Ehr. 
= C. costatus, Sm. ; Cymatopleura solea, Sm. ; C. elliptica, 
Breb. ; and Surirella ( Cymatopleura ) plicata, Ehr. (‘ Mi- 
crogeo.,’ T. xv. A., figs. 50, 51), very large and fine. A 
figure is given in Dr. Carpenter’s work on the ‘ Microscope ’ 
of this last species (p. 313, 4th ed.), copied from Ehrenberg. 
Specimens of this form occur in the gathering in every stage 
of gradation between Ehrenberg’s figures and Professor 
Smith’s figure of Cymatopleura Ilibernica, which seems to 
have been delineated and described from small specimens 
slightly contracted below the extremities. There can be no 
doubt that the two forms are one and the same species, which 
ought, therefore, to be designated Cymatopleura p>licata, 
Ehr. The extremities of this species seem to be subject to 
considerable variation, such as is witnessed in different speci- 
mens of C. solea, Sm., and led him to constitute anew species 
of the apiculate specimens, C. apiculata, Sm. — a distinction 
which ought never to have been made. I cannot agree with Mr. 
Ralfs (‘ Prit. Inf.,’ p. 793) and Rabenhorst (‘ Europ. Diatom.,’ 
p. 60) in referring C. plicata to C. elliptica. In none of my 
own gatherings, except from Wallington and the River 
Coquet at Elyhaugh, have I ever met with C. plicata. In 
