WALKER-ARNOTT, ON MARINE DIATOMS. 175 
of no one who has paid more attention to the species of Hpi- 
themia than Mr. Comber, and much wish that he would give 
to the public a monograph of them. He is the only one who 
has studied with care, not merely the F.V. and S.V., but 
also the vertical view, corresponding to a transverse section 
of the frustule, which yields characters of some importance. 
Tryblionella gracilis is stated to be from fresh and brackish. 
water. It usually occurs in the latter, and these specimens 
are much larger and finer than the others; but it certainly 
is found in various localities at a considerable elevation above 
the sea. Indeed at one time Smith considered this form dis- 
tinct, and named it 7. minor ; it is more oval than the littoral 
form, and is scarcely distinguishable from T. levidensis, except 
by the finer striation. Dr. Gregory has published a 7. apicu- 
lata,* which he had from Cumbrae, and felt doubtful whether 
to consider a marine or fresh-water species. This is very 
common on all the coasts of Scotland and England, and was 
considered by Smith, when sent to him by me several years 
ago, a slate of Nitzchia dubia, var. 3. It is alluded to in 
‘Micr. Journ.,’ mi, p. 809. It is certainly a brackish-water 
or marine species. 
Navicula Smith (elliptica, Sm. ‘Syn.’) and N. elliptica, 
K. (ovalis, Sm. ‘Syn.’) are very closely allied, and would 
have been probably united, were the one not peculiar to 
fresh water, the other to the sea; the striation of the 
former is coarser. Now, taking striation as our guide (and 
here I regret to differ from Smith, and think striation, or 
any microscopic difference, unsupported by more natural 
characters, as a very fallible one), 1 have been supplied by 
Dr. Dickie with a series of gatherings made last December 
in acave at Skateraw, on the Kincardine coast, which are 
very puzzling. I understand the cave to be far beyond the 
reach of the tide, although during great storms the spray 
may be carried to it. Near the entrance of the cave is 
N. Smithii, accompanied by Orthosira spinosa and Himan- 
tidium gracile. A little further in are N. Smithit and 
N. elliptica, much mixed, and Orthos. spinosa; further in, a 
gathering was made composed of N. elliptica, Epithemia ru- 
pestris, and E. Westermannit, Sm., so varied and mixed as to 
be undistinguishable ; Cocconeis Thwaitesii, Denticula obtusa, 
Odontidium Tadellaria and mutabile (both small). I believe 
that the third sample was got on the opposite side of the cave 
from the second, but am not quite certain; I have not ob- 
served in it either N. Smithii or Orth. spinosa. At the 
further extremity of the cave Orth. spinosa was got quite 
* «Trans. Micr. Soc.,’ vol. v, p. 79, tab. i, fig. 43. 
