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by an intervening membrane and constructed of pure 
. Silica, whose siliceous loricze are beautifully sculptured so 
that they present some of the most elegant objects which the 
microscopist has to contemplate. They vary in outline 
almost indefinitely, but in general those which inhabit the sea 
are constructed on a different plan from those which grow in 
fresh water. Hence the observer, who has made these 
organisms a study, is able to distinguish forms which have 
lived, grown and reproduced, subject to either of these two 
conditions. In fact, as I have devoted a very large part of my 
time for the last fifteen years, at least, to the careful study 
of the Diatomacez under varying circumstances and from 
many localities, and more particularly as relating to geology, 
I may say that I am almost prepared to determine by the 
examination with the microscope alone of a specimen from 
such a deposit as to whether it has been formed in a lake, a 
pond, or a river, at what altitude, approximately the size of 
the lake or the swiftness of the stream, besides other facts 
connected with its deposition. However, onall of these points, 
I am not sufficiently certain as yet to warrant any further 
dwelling upon them here, but hope, as further collections 
are made and records gathered to obtain more valuable 
elucidating information in this connection. The circumstances 
connected with the mode of growth and, in fact, the life- 
history generally of these minute organisms has been by no 
means carefully enough studied, but I have been for years 
gathering data relating to the family which will, I trust, assist 
very materially in unraveling some complicated and impor- 
tant problems in Biology. 
The Diatomacez, then, being constituted, as I have said, of 
essentially two parts, or “valves” as they have been called, 
united by means of a hoop or membrane likewise siliceous in 
composition, increase or grow by a process of subdivision, in 
such a way that between and midway of the two valves, are 
formed two new ones, so that the original single individual 
becomes in time two united individuals. This double indi- 
vidual, or really two individuals may, at once, separate into 
two, or remain united, but, at all events, the process of sub- 
