O’Mrara—Report on the Irish Diatomacee. 245: 
of the globe; some the accumulations of marine, others of fresh 
water growth. Among these latter, the Irish deposits of Lough 
Mourne, Lough Islandreavy, Toombe Bridge, and Tollymore 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. Nearly all the species contained in 
these various deposits have been found living at the present day; and it 
isa 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. Kitton, 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 differmg from the 
specimens of the same species I had found living a few days before, 
in a ditch not far from my residence in the county Dublin. Another 
deposit discovered by Dr. Moss, R.N., at Vancouver’s Island, was 
sent to me for examination; and in it, among many other well-known 
forms, I found in great number, specimens of Nayicula Americana, in 
all respects identical with forms of that species collected by my friend 
the Rev. George Davidson, from a deposit at Lough Canmore, in the 
north of Scotland, and those I had myself gathered some time ago in a 
living state on the borders of Lough Neagh. Count Castracane is 
of opinion that Diatoms must have existed even in the remote ages of 
the Paleozoic 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 naturalist was furnished by Professor Mici, who considered it 
to belong unquestionably to the Miocene period. ‘This result is-con- 
firmed by the statement of Pfitzer, that all the fresh water, as well as 
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 
