178 WALKER-ARNOTT, ON MARINE DIATOMS. 
What Smith had in view is S. parvula, Kiitz. (from which 
S. parva, Kiitz., from De Brébisson, scarcely differs; while 
S. parva from the Gulf of Venice appears to be S. énvestiens, 
Sm.) The S. fasciculata of Kiitzing seems to be a common 
variety of S. affinis with blunt valves; Agardh’s Diatoma 
fasciculatum is partly S. affinis and partly S. arcus of Sm. 
(not Kiitz., which seems to be a state of S. Gailloni) ; while 
Exilaria fasciculata, var. a, of Greville, is S. radians, and his 
var. (3 a mixture of S. arcus, S. affinis, and S. pulchella. 
Doryphora Boeckii is stated to be marine; but Smith wrote 
me afterwards that he had doubts on the subject ; when I 
have found it in Arran, it was growing intermingled with 
Synedra pulchella, S. minutissima, and Eunotia (also a species 
of Synedra) arcus, Sm., all of which were reputed fresh- 
water species. I have just mentioned that the first two are 
found at the mouths of rivers or streams, but not beyond 
high-water mark, and D. Boeckit is probably in the same 
predicament; I have, however, specimens from Norway 
(from Mr. Norman, of Hull) in which it is mixed only with 
marine species. 
The last to which I shall at present allude is Homeocladia 
filiformis: this is said to be from brackish water, and it is occa- 
sionally found there; but it grows abundantly on fresh-water 
algeze in the Monkland Canal, near Glasgow, a canal which 
is at the elevation of about 165 feet above the level of the sea, 
and to which sea-water cannot possibly have access. I cannot 
call the water there either sweet or fresh, for it is both dirty 
and offensive to the nasal organs; but it contains no salt. 
It thus appears that although the statement made by 
Smith, and quoted at the commencement of this paper, may, 
on the whole, be correct in theory, it is of very little prac- 
tical value, and may lead incautious observers to very in- 
accurate conclusions in the distinction of species. At pre- 
sent, and until more facts are collected, it may be held a 
safe rule that when a whole genus is peculiar to fresh water 
with the exception of a single species, and that exceptional 
species is accompanied by some fresh-water forms, it ought 
to be held also as belonging to fresh water, or at most to 
brackish water; and that when the genus is marine, with 
the exception of one species, this, if accompanied by any 
properly ascertained marine or brackish-water Diatoms, may 
be regarded also as a marine or brackish-water species: and 
although a mere gathering may not clear up the point when 
ditches, canals, and sluggish streams are in question, a slight 
inquiry as to how these are fed may contribute much 
towards it. 
