104 Gregory, on Fossil Diatomacece. 
Observations on some Deposits of Fossil Diatomace^. By 
William Gregory, M.D., F.R.S.E. 
(Read April 19tli, 1854.) 
In the series of microscopic objects issued by the Zurich 
Microscopical Association, there occurs a specimen of Berg- 
mehl, stated to be from Lillhaggsjon in Lapland, which is 
very remarkable in several particulars. 
First, there is a very great abundance of Eunotia Triodon, 
exhibiting the most astonishing variations of outline, so that 
the extreme varieties in opposite directions, those, for ex- 
ample, which are short, compressed, and have strongly-marked 
prominences, and such as are long, flattened, the apices being 
lengthened out, while the prominences actually disappear, or 
can only be traced by a hardly-perceptible waviness in the 
dorsal outline, would hardly be supposed to belong to the 
same species, and yet a perfect and gentle gradation may be 
traced from the one extreme to the other. This remarkable 
tendency to vary in form is peculiar, among the Eunotice I 
have seen, to this species, E. Triodon and to E. hegehha, KUt- 
zing. It is totally absent in the common fossil forms of E, 
Tetraodon and E. Diadema, which hardly vary at all, save in 
size. 
It appears to me that this fact, especially when we consider 
that all these species often occur together, as, for example, in 
the Mull deposit, where E, Triodon^ though not frequent, is 
just as variable as in the Bergmehl under consideration, 
demonstrates that these species are really distinct, and not, as 
some have conjectured, varieties of one, which may present 
one, two, three, four, five, six, seven, or more prominences. 
If all belonged to one species, all should be alike variable or 
alike constant, whereas some vary ad infinitum, others not at 
all, in form at least. Nor can it be said that such a form as 
E. Triodon is developed, as a variety, only under certain cir- 
cumstances ; for in the Mull deposit it occurs with all its 
peculiarities, and therefore the supposed circumstances must 
have occurred ; and yet, in that deposit, E. Tetraodon and 
E. Diadema are much more abundant, and show no tendency 
to vary in form. But these two last-named species are absent 
from this Lapland deposit, where E. Triodon abounds. It 
seems to me that these facts settle the question as to the 
species named, which must be held to be true and well- 
marked species ; one of the characters of E. Triodon and of 
E. begehha being a tendency to vary in form, while fixity of 
form characterises E. Tetraodon and E. Diadema. 
This Laponian deposit also contains E. serra, and I think 
