O'Meara — Report on the Irish Biatomacece. 245 



of the globe ; some the accumulations of marine, others of fresh 

 "water growth. Among these latter, the Irish deposits of Lough 

 !Moume, Lough Islandreavy, Toombe Ericlgc, and ToUymore Park, 

 are distinguished for the number and beauty of the species they con- 

 tain : and we are indebted to the industry and intelligence of Mr. 

 Gray, of Belfast, for the discovery of several sub-peat collections in 

 various parts of the country. jSTearly all the species contained in 

 these various deposits have been found living at the present day; audit 

 is a noteworthy fact, that the forms of these numerous species, however 

 remote from one another in time and space, exhibit no appreciable 

 divergency. As an illustration I may mention a few facts. Through 

 the kindness of Mr. Kjtton, of Norwich, I was supplied with a sample of 

 a fresh water deposit from California, which contained numerous speci- 

 mens of Synedra amphirhynchus, in no respect diffeiing from the 

 specimens of the same species I had found living a few days before, 

 in a ditch not far fi-om my residence in the county Dublin. Another 

 deposit discovered by Dr. Moss, E. IS"., at Yancouver's Island, was- 

 sent to me for examination ; and in it, among many other well-known 

 forms, I found in great number, specimens of Navicula Americana, in 

 all respects identical with forms of that species collected by my friend 

 the Rev. George Davidson, fi'om a deposit at Lough Canmore, in the 

 north of Scotland, and those I had myseK gathered some time ago in a 

 living state on the borders of Lough jS'eagh. Count Castracane is 

 of opinion that Diatoms must have existed even in the remote ages of 

 the Palaeozoic period. It remains to be proved whether this was so 

 or not ; but in his researches in the lignite formation of Urbino he has 

 traced existing species so far back as the earlier epoch of the Tertiary 

 formation. The specimen of lignite examined by this distinguished 

 Italian natm-alist was furnished by Professor Mici, who considered it 

 to belong unquestionably to the Miocene period. This resiilt is con- 

 firmed by the statement of Pfitzer, that all the fresh water, as well aa 

 marine forms hitherto discovered in the deposits of the Tertiary 

 period, belong to existing genera and species. The generations of a 

 Diatom in the space of a few months far exceed in number the genera- 

 tions of man from the earliest time to the present day ; and yet 

 we find that the individuals now living retain without alteration the 

 characteristics which distinguished the species at the remotest time to 

 which their existence can be traced. It might be alleged in this case 

 that the silicious valves within which the valves of successive genera- 

 tions are developed necessarily impress the characters of the parent on 

 the offspring ; and that, therefore, any tendency to variation, however 

 powerfully it might operate, would be checked by the irresistible 

 force of external pressure. But the sporangia before the soft skin 

 has become solidified by the secretion of silex are of a more plastic 

 character, and afford a facility for variation if the cell-contents were 

 endowed with any such tendency. And although the formation of 

 sporangia has been observed in but very few instances, yet the 

 frequent recurrence of this process of reproduction is forced on our 



