446 



atium Favus, Acttnocyclus undulatus, Coscinodisciis radiatus, se- 

 veral Grammatophorse, and various others. 



In short, I have no hesitation in saying, that I believe all the 

 forms in the ^gina Clay Marl, which is the oldest Diatomaceous 

 deposit yet described, will be found living on our coasts. 



It may also be observed, that of all the forms Bgured by Ehren- 

 berg from more recent strata, whether miocene, like the bed on which 

 the town of Richmond, Virginia, is built, and several kinds of Berg- 

 mehl, or pleiocene, like other Bergmehls and polishing slates, &c., or 

 still more recent, the great majority are perfectly identical with 

 existing Diatoms, 



Indeed, although many forms are stated in Ehrenberg's earliest 

 writings to be fossil only, and have been supposed to be extinct, 

 the progress of observation is continually adding to the number of 

 species which are found also in the recent state. Thus, for ex- 

 ample, the whole group of dentate Eunotiaa, which abound in the 

 Lapland and Finland Bergmehls, were long thought to be only fos- 

 sil. But they have been nearly all found living in America, and I 

 have myself seen several of them recent in this country. Eunotia 

 triodon, long supposed to be extinct, occurred scattered in many of 

 the Scottish fresh-water gatherings I described in this place three 

 years ago, and I found it this last summer the predominant form in 

 a gathering brought from Arran by Dr Balfour. 



Taking these facts into consideration, I am led to believe that 

 we have no evidence that any species of Diatom has become extinct, 

 as so many species, and even genera and tribes, of more highly or- 

 ganized beings have done. I observe that Mr Brightwell expresses 

 a similar opinion in his valuable paper on Chaetoceros. (See Micr. 

 Jour. IV. 105.) His anticipation that Syndendrium would be found 

 recent has been fulfilled. (See List, p. 450.) Our knowledge of the 

 existing species is yet very imperfect, as is obvious from the facts 

 adduced in this paper, in which so many undescribed forms are 

 shown to exist in a kw localities of one estuary. And among these 

 are such forms as the present one, N. pratexta, which has hitherto 

 been supposed only to exist as a fossil. 



It is well known, that in certain animal tribes, Molluscs, for ex- 

 ample, many species are common to the present and to earlier geo- 

 logical periods. I need only allude to the circumstances that the 

 eocene, miocene, and pleiocene strata are named from their propor- 



