The
almost total absence of limestones in the formation may be regarded
as a sufficient explanation of the fact that the _Foraminifera_
are not more largely and unequivocally represented; though the
existence of greensands in the Cambrian beds of Wisconsin and
Tennessee may be taken as an indication that this class of animals
was by no means wholly wanting. The same fact may explain the
total absence of corals, so far as at present known.
[Illustration: Fig. 29.--A portion of _Oldhamia antiqua_, Lower
Cambrian, Wicklow, Ireland, of the natural size. (After Salter.)]
The group of the _Echinodermata_ (Sea-lilies, Sea-urchins, and
their allies) is represented by a few forms, which are principally
of interest as being the earliest-known examples of the class.
It is also worthy of note that these precursors of a group which
subsequently attains such geological importance, are referable to
no less than three distinct _orders_--the Crinoids or Sea-lilies,
represented by a species of _Dendrocrinus_; the Cystideans by
_Protocystites_; and the Star-fishes by _Palasterina_ and some
other forms. Only the last of these groups, however, appears
to occur in the Lower Cambrian.
[Illustration: Fig. 30.--Annelide-burrows (_Scolithus linearus_)
from the Potsdam Sandstone of Canada, of the natural size. (After
Billings.)]
The Ringed-worms (_Annelida_), if rightly credited with all the
remains usually referred to them, appear to have swarmed in the
Cambrian seas.
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