88 THE MEDALS OF CREATION, Crap. VER 
by separating transversely, and leaving each cell or Srustule 
attached at the angles, produce those beautiful chains of 
angular green transparent cases, so constantly seen under 
the microscope when substances from fresh-water streams or 
lakes are submitted to examination. 
As many of these forms are endowed with spontaneous | 
motion, and possess other properties common to animal 
organization, it is not surprising that their vegetable nature 
was doubted, and that even so profound a naturalist as M. 
Ehrenberg placed them in the animal kingdom : the greater 
number being comprised in his family of Bacillaric, were 
described in the former edition of this work, as Infusoria or 
Animalcules ; in conformity with the classification of the 
illustrious microscopist, whose splendid works and indefati- 
gable labours have so greatly promoted the advancement of 
microscopical investigation.* 
These minute vegetable organisms are placed by botanists 
in two tribes, the Diatomacee or the Brittle-worts, and the 
Desmidiee. The latter are exclusively inhabitants of fresh- 
water, while a large proportion of the former are marine 
plants. Some naturalists (M. Brébisson) restrict the name 
Diatomacee to those species which secrete siliceous enve- 
lopes ; and that of Desmidiec to those whose structures are 
not siliceous, and are reducible by heat to carbon. As the 
durable parts of these plants alone concern the geologist, the 
name Diatomacee will be employed as a general term in 
reference to their fossil remains. : 
These tribes of Algze abound in every lake and stream of 
fresh-water, in every pool or bay, and throughout the ocean 
from the equator to the poles. Certain kinds of sea-weeds 
secrete carbonate of lime; but the Diatomaceze have the 
power of separating silex, or the earth of flint, from the 
* The whole of the objects called Infusoria in the first edition 
of “The Medals of Creation” belong to various kinds of Diato- 
macee. 
