NEW FORMS OF MARINE DIATOMACE^l. 483 



clypeus, found in the polishing slates of Bohemia, and the whole series of den- 

 tate Eunotia?, 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 Eunotice, such as E. Diadema, E. heptodon, E. octodon, 

 E. Serra, and others, have been found recent. I have myself often found, during 

 the last two years, E. triodon, 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 Balfour, 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 N. prcetexta, 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 Ehrenberg'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 



