NEW FORMS OF MARINE DIATOMACER. 483 
elypeus, found in the polishing slates of Bohemia, and the whole series of den- 
' tate Eunotic, found so abundantly in the Lapland Bergmehls. 
But the progress of observation has shown that these forms are still in exist- 
ence. C. clypeus has recently been found in British waters ; and in America, and 
elsewhere, the dentate Eunotie, such as E. Diadema, E. heptodon, E. octodon, 
£. Serra, and others, have been found recent. Ihave myself often found, during 
the last two years, E. iriodon, a form long regarded as extinct, in many of our 
streams, although scattered. But last summer I detected it as the predominant 
form in a gathering made by Professor Batrour, in a small stream on a hill in 
Arran, not far from Lamlash. 
I conclude, therefore, that our knowledge of the existing species of Diatoms 
is yet far too limited to allow us to say that any fossil species no longer exists. 
In this very paper, I make known the actual existence of several species, hitherto 
supposed to be exclusively fossil, and every day adds to the number of existing 
forms, while it diminishes that of those conjectured to be extinct, few of which 
are now left. Surely, when one or two localities yield so many undescribed forms 
as I have here the honour to lay before the Society, we are not entitled to con- 
clude that any form is extinct, because hitherto it has only been met with in the 
fossil state. In the present state of our knowledge, it is far more probable, that 
we shall ultimately find, as I have done in the case of NV. pretexta, that the sup- 
posed extinct species are all still in existence. 
But, it may be asked, How is it that you suppose no species of Diatoms to have 
become extinct, when, in almost every other class, the extinct species far out- 
number the existing ones? In answer, I would observe, first, that we have 
no undoubted evidence of the existence of Diatoms earlier than the Clay Marl 
above named, which is either Eocene, or a member of the latest Chalk deposits. 
Now, if it be Eocene, then we know that that formation contains, even among 
fishes, a certain proportion of existing species. This proves that the condition of 
the Eocene period did not differ nearly so much from the present conditions as 
- those of earlier deposits must have done; those, for example of the Carboniferous 
series, of the Old Red Sandstone, or of the Silurian strata. 
Secondly, the size of Diatoms is so very minute, and their structure so exceed- 
ingly simple, that they must be little, if at all, affected, even by very considerable 
climatic variations. Of this, indeed, we have ample evidence, so far at least as 
concerns existing differences of climate. If we consult the plates of EnrEnBERG’s 
Microgeology, we shall see that the existing species of Diatoms found in the most 
distant and different parts of the world, in the Arctic and Antarctic Seas, in the 
tropical zone, and in our own temperate regions, are, for the most part, absolutely 
identical. There are, no doubt, local differences; but these, as is shown in this 
paper, may be very great in almost contiguous localities. On the other hand, 
having examined the diatoms in a large number of American and other exotic 
