EULENSTEIN, ON HOM@CLADIA IN FRESH WATER. 173 
athin cover in adrop of distilled water, I submitted it to heat, 
and now readily observed undoubted Nitzschie lying within 
the scorched tube. Ata point where the latter was ruptured 
some frustules had escaped and allowed the closest examina- 
tion, from which I am inclined to consider the new fresh- 
water form a near ally to H. fitfililiformis, W.Sm. (3 minor, and 
to pronounce it even a mere variety of the latter species. The 
frustules are somewhat smaller andthe frontview broader; their 
arrangement is rather crowded than fasciculate, the filaments 
observed were undivided—characters either not sufficiently 
important, or not sufficiently established for the present, to 
warrant a separation on these grounds. A figure was thought 
to be unnecessary, the more as [ shall take pleasure in present- 
ing the original specimen described to the collection of the 
London Microscopical Society, and I shall be glad to com- 
municate a few left to those especially interested.* 
I may, with propriety, conclude this account with an allu- 
sion to the fact that the above instance of the apparent 
occurrence of a marine species in fresh water is not an isolated 
one in the class Algze. I only mention at present another 
diatom, Nitzschia dubia, W. Sm., which being indigenous to 
brackish water, seemingly occurs in several localities on the 
continent in springs and ditches. As I am preparing, for 
insertion in the present publication, a full report on such 
eases, I shall be thankful to receive any opinions bearing on 
the subject, from British diatomists, who, from their ready 
access to the seashore, are well enabled to form a correct 
judgment on the following practical question which has often 
occurred to me in connection with the facts mentioned, and 
which seems worth while discussing at a future opportunity : 
—‘ Are forms apparently identical, but hving in opposite 
mediums, to be regarded as one species, or does the medium 
constitute the hmit in such cases?” 
* The author will moreover be happy to exchange rare Continental 
species and deposits for British, marine, and brackwater gatherings, or 
foreign deposits. 
