WALKER-ARNOTT, ON MARINE DIATOMS. 171 
by ocean currents from a very remote, shore. Even fresh- 
water species are deposited in that way. I have found, 
while dredging in fifteen or twenty fathoms, off the coast of 
Arran, at the mouth of the Clyde, Eunotia triodon, Himan- 
tidium pectinale, and Navicula rhomboides; and frustules 
of the last two have also occurred to me in rocky pools near 
low-water mark. Dr. Dickie* has given a list of several 
fresh-water species which he met with m Aberdeen Bay, in 
the stomachs of Ascidia, in from twenty-five to thirty fathoms, 
and at five to six miles from land: among these are Himan- 
tidium pectinale, Tabellaria flocculosa, Cocconeis pediculus, 
&e. I have occasionally seen detached valves of Orthosira 
arenaria in dredgings; but, deceived probably by the 
locality, Dr. Gregory, in his paper on ‘ Clyde Diatoms,’ fig. 
44, has reproduced it as a new species of Melosira (?) or 
Coscinodiscus (?), he is uncertain which. 
The mere finding of a species in the sea is therefore no proof 
of its having grown there; so that other means must be resorted 
to before asserting that such specimens must form a species 
distinct from an analogous one occurring in Alpine streams, 
or elsewhere in fresh water. 
Lewes, in Sussex, where Smith principally made his 
observations, is peculiarly unsuited to solve these difficulties. 
The influence of the tide is there conspicuous, and when a 
species is rare, and confined to only one locality, it is quite 
impossible to deduce whether it is a brackish-water one, or 
a marine form carried there by the tide, or a fresh-water 
one swept down from the higher grounds. When, therefore, 
only one or two localities are indicated by Smith, and 
these entirely about Lewes, or in a similar place, the pecu- 
liarity of the water most congenial to that Diatom must be 
held to be still m doubt. A few instances of this may be 
mentioned. 
Nitzschia dubia is stated by Smith to occur only in brack- 
ish water. I have it from the Queen’s Park, Edinburgh, and 
from several places about Glasgow, South Wales, &c., at a 
considerable altitude above the level of the sea; so that, 
although seemingly to grow equally well where the water is 
partly saline, it certainly affects truly fresh water. Nitzschia 
Brebissoni is stated by Smith to be found in fresh water (he 
. found it only at Lewes) ; but his slides contain many marine 
species (as Nitzschia sigma, Navicula minutula, Tryblion. scu- 
tellum, gracilis and punctata, Bacillaria paradoxa, &c.) as well 
as fresh-water ones (as Nitzschia sigmoidea, Navicula cuspidata, 
Nav. ambigua, &c.) Smith drew his conclusions from a. 
; * « Annals of Nat. Hist.’ for 1848, vol. i, p. 324. 
VOL. VII. he 
