362 
THE GEOLOGIST. 
We proceed now to detail the chief characters of each of the 
above great groups, beginning with tlie lowest, and the type-plans 
on which they are constructed. To say what is the lowest form of 
animal-life is indeed difficult, as it is also of the vegetable. We 
have certain moving and apparently living cells and frustules, such 
as the monads, diatoms, and other organisms, commonly termed 
(from their usual presence in vegetable infusions and stagnant 
water) Infusoria ; but the discussion is still strong as to their proper 
position, although the stronger evidence is at present on the side of 
those who group them with the vegetable kingdom. At all events 
the minutest and simplest forms of organized objects are simple 
cells ; and as all animals and vegetables, whatever their rank, are 
built up of an organic structure composed of cells, it is at least 
difficult to determine either the identity with each other of such 
primitive cells ; the commingling in them of the rudimentary 
stages of both kingdoms ; or from their smallness, delicateness, and 
similarity of chemical composition and their structural resemblances, 
to point out the essential distinctions. 
These primitive cell-forms, too, possessed of no solid parts, and 
liable to almost instant decay after death, enter not into the domains 
of palaeontology ; and it is only in the case of the diatoms and 
siliceous or calcareous loricated (shelled) forms that we find any 
traces in a fossil state. These, however, are found in such abund- 
ance in some of the later Tertiary deposits as to form whole beds, 
which are, as in the case of the Tripoli and the Berg-mehl, used for 
industrial or domestic purposes. But as these belong, according to 
most authorities, to the vegetable tribes, we shall notice them more 
fully under that head and when we come to treat in the progress of 
our work of the Tertiary rocks. 
The first class then which we notice of the Protozoa are the 
Rhizopoda or root-foot animals. The Protozoa are all more or less 
globular or amorphous ; for even the sponges or Porifera have 
generally a spherical shape, although in some genera of this 
higher group the true globular form is indented into fannel- 
shaped cavities or elongated into tubular stems. In the lower 
gi'oup rccog-nized in the fossil-state by the innumerable shells of 
Foraminifera, abundant in many rocks, but best known in the 
