244 Proceedings of the Royal Irish Academy. 



■united in short filaments ; and Brebissonia Boeckei = Doryphora 

 Boeckei, in which the friistiales are stipitate, are notwithstanding 

 these peculiarities of growth included as species under the genus 

 [N'aYicula. If Smith and others attached too much value to these sub- 

 ordinate features, and therefore separated the forms which exhibited 

 them very far fi'om the JSTaviculese, 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 I^aviculese. 



On the Distrilution of the BiatomacecB. 



Some species are found only in fi'esh water, some only in salt 

 water, while others select as their nonnal habitat places in which salt 

 and fi'esh 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 

 Diatomacese 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 solidifled into rock, the exuviae 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 artiflcial agencies, they may be extracted, and subjected 

 to inspection ; and if found in sufiicient 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- 

 macese, 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 

 Diatomaceoe, 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 



