DESMIDS AND. DIATOMS. 595 
the flint-nodules of the chalk formation, and within a still 
more recent period similar forms have been observed in 
the analogous hornstones of the Devonian and Silurian 
ages, associated in this latter case with unequivocal Dia- 
tomaceous shells. As regards these Xanthidia, whieh 
have usually been regarded as remains of Desmids, it is 
certainly singular that, while all recent Desmids are 
purely fresh water, these should occur in marine deposits : 
and secondly, that, destitute as they are for the most part 
of the silicious shell of the Diatoms, they should occur in 
a fossil state at all. Yet the resemblance is certainly a 
striking one, and their occurrence with the kindred Dia- 
tomaces throws some degree of plausibility upon this be- 
lief. However this may be, the existence of one group 
at least of these organisms being established for these 
early periods, we can scarcely doubt that their numbers 
were as great then as in the seas of our own day, and 
that they have been present through all the great geolog- 
ical ages, even though metamorphism and other agencies 
have for the most part obliterated all traces of their beau- 
tiful but fragile shells. It is highly probable that accom- 
panying the lower forms of animal life, these humble 
types of vegetation were among the first introduced upon 
the globe, performing then, as their representatives now 
do in the arctic seas and at great depths, where the 
higher forms of vegetation are wanting, the part of puri- 
fying the waters, as well as of contributing food for the 
sustenance of the different forms of animal life with which 
they were associated. 
Thus we see that the lower no less than the higher 
forms of life have their appointed place, each fulfilling its 
own part, and each worthy of the admiration and the 
study of the observing mind. 
