244 Proceedings of the Royal Irish Academy. 
united in short filaments; and Brebissonia Boeckei = Doryphora 
Boeckei, in which the frustules are stipitate, are notwithstanding 
these peculiarities of growth included as species under the genus 
Navicula. If Smith and others attached too much value to these sub- 
ordinate features, and therefore separated the forms which exhibited 
them very far from the Naviculez, with which, as regards the general 
structure of the frustules, they are intimately related, Heiberg, on 
the. other hand, I consider, has made a mistake in ignoring these 
peculiarities altogether. Recognising these various normal modes of 
growth as generic distinctions, I have included the forms as separate 
genera of the group Naviculeex. 
On the Distribution of the Diatomacee. 
Some species are found only in fresh water, some only in salt 
water, while others select as their normal habitat places in which salt 
and fresh water habitually or occasionally commingle. I have 
indeed frequently found fresh water species in the stomachs of 
Ascidians dredged from a considerable depth in the sea; but their 
occurrence therein indicates the influx of fresh water in the immediate 
neighbourhood. And when marine forms are found in fresh water, as 
occasionally they may be, they indicate that the place is within. the 
range of tidal influence. ; 
An experienced observer will be able at a glance to ascertain 
whether a gathering is marine, or made in fresh or brackish water ; 
and not only so, but will be able to discriminate the lacustrine and 
alpine forms from those incidental to other situations. 
It is not possible to ascertain for what period the life of the 
Diatomacee continues, but when their course, be it long or short, is 
ended, the silicious covering sinks into the sediment: and when in 
the process of ages the sediment is solidified into rock, the exuvie of 
the Diatoms that lived in the water during the period of deposition 
continue unaltered in their stony shroud. If the rock be decomposed 
by natural or artificial agencies, they may be extracted, and subjected 
to inspection; and if found in sufficient number, the species discovered 
may serve to illustrate the circumstances under which the deposit was 
formed. 
Irrespective of the variety and symmetrical beauty of the Diato- 
macez, there is another circumstance which invests them with a pecu- 
liar interest: it is this, that no existing organism, whether it be 
vegetable or animal, can boast of so ancient a lineage. Countless have 
been the genera and species of living beings which flourished during 
the several geological periods, and of which no representatives survived 
the vicissitude which brought their epoch to a conclusion; but so far 
back in the annals of the earth as research has been able to trace the 
Diatomacez, the species which have been discovered are identical with 
those we have living at the present time. Numerous are the fossil or 
subfossil diatomaceous deposits which have been discovered in all parts 
