HUTTON, ON ARACHNOIDISCUS. 133 
fasciola, P. littorale, Denticula (sp. ?), Melosira nummuloides, 
and a few frustules of Biddulphia aurita. Ten of these, according 
to Smith, inhabit the sea, six either the sea or brackish water, 
and two brackish water only. I think that these two frustules of 
Arachnoidiscus must be considered bond fide Irish specimens, 
and not adventitious, for the following reasons :—I have never 
worked at, or had in my possession, any guano or fossil 
diatomaceous earth ; but in February last I had a small piece of 
sea-weed from China given to me, from which I obtained 
about a dozen specimeus of LH. Ehrenbergii, but none of A. 
ornatus. 
Luckily it so happened that both the glass slide and the 
dipping-tube which I used in examining the gathering from 
Malahide were quite new, and had never been used before. 
The bottle also into which I collected these diatoms never had 
had anything but fresh-water diatoms, &c., init. The only 
other thing that came in contact with the gathering was the 
test-tube in which it was prepared, and in which I might, 
perhaps, have boiled the sea-weed from China which I have 
before mentioned; but even if so, the test-tube has been in 
constant use ever since for preparing other, chiefly fresh- 
water, British diatoms, and had been washed out and cleansed 
dozens of times since the Arachnoidisci were in it. No rag of 
any sort had touched that first portion of the gathering in 
which I found A. ornatus, and of course I took the precaution 
of using an entirely new one in examining the rest of the 
gathering. I have carefully examined the test-tube since, 
and am quite satisfied that no others remain in it, as so 
large and distinct a diatom is easily recognised with an inch 
object-glass. 
Taking, therefore, into consideration the fact that a frus- 
tule of H. Ehrenbergii was found by M. de Brébisson, 
actually, I believe, attached to a specimen of Sphacelaria 
olivacea, Ag., sent to him from Ilfracombe by Mr. Ralfs, it 
appears to me more probable that these specimens of mine 
are really British than that the two frustules should have 
remained in the test-tube for ten months, notwithstanding 
repeated boilings and washings, and should have at last 
come out both together with a quantity of other diatoms 
whose habitat is precisely similar to their own. 
