O’Mrara—Report on the Irish Diatomacee. 241 
frustules were poured; the prolongations became gradually more and 
more constricted at the base, until ultimately they were completely 
cut off from the mucous sac, in which the frustules remained folded, in 
a state of perfect inanition. The process described was completed 
within the space of a few hours, so that in innumerable instances I was 
able to trace it from beginning to end—that is, from the commence- 
ment of conjugation up to the formation of the sporangia. Smith refers 
to cases of Cocconema cistula, and also of Synedra radians, haying been 
found aggregated in great numbers and enclosed in mucous sacs simi- 
lar to what has been described in the case of Diatoma vulgare; and 
all three cases seem to me to represent the same phase of conjugation : 
and I am disposed to think that, as in Diatoma vulgare, so in the 
other two cases, the encysted frustules were not, as Smith considered 
them, young frustules in course of development from a sporangium, 
but parent frustules preparing to produce sporangia. 
Instances of conjugation in any of its varied forms are rarely to be 
met with. When Smith published his Synopsis, in 1856, cases had 
been observed in thirty species, included in seventeen distinct genera ; 
and during the interval of fifteen years that had elapsed when Pfitzer 
published his work, ‘‘ Uber Bau und Entwicklung der Bacillariaceen,’’ 
only twenty-eight cases had been added to the list, exclusive of that 
of Diatoma vulgare, making a total of sixty-one. This remarkable fact 
Smith thus endeavours to account for: ‘‘ During conjugation the process 
of self-division is arrested, the general mucous envelope or stratum 
produced during self-division is dissolved, and the conjugating pairs 
of frustules become detached from the original mass; they are thus 
more readily borne away and dispersed in the surrounding currents, or 
by the movements of worms or insects, and their detection becomes in 
consequence more casual and difficult.” It is not improbable, however, 
that the mode of collecting, and the time that is often suffered to elapse 
before the collection is submitted to investigation, may have more to do 
with the fact. And, in confirmation of this view, I would mention 
that, although I have for very many years been engaged in the study 
of the Diatomaceze, and have made innumerable collections at all 
seasons of the year, I have not been so fortunate in observing instances 
of conjugation as some friends whose collections have been made with 
a view to the discovery of other organisms. Their gatherings are 
usually made in large bottles containing a considerable quantity of 
water, by which the specimens may be preserved for a long time in 
their normal state—my gatherings being put up in minute bottles with 
little water, so that the vigour of the frustules is greatly abated be- 
fore an opportunity of examining them may be afforded. As to the 
seasons of the year in which conjugation is most likely to occur, the 
facts hitherto accumulated do not afford much information. Besides 
the case of Diatome vulgare which I observed in conjugation in the 
month of August, seventy-two observations, with specification of date, 
have been recorded, making seventy-three in all. Of these, twenty- 
three occurred in spring, twenty in summer, twenty-four in autumn, 
