Lastly, in the fossils
known as _Receptaculites_ and _Ischadites_ we are also presented
with certain singular Lower Silurian Protozoans, which may with
great probability be regarded as gigantic _Foraminifera_. Their
structure is very complex; but fragments are easily recognised
by the fact that the exterior is covered with numerous rhomboidal
calcareous plates, closely fitting together, and arranged in
peculiar intersecting curves, presenting very much the appearance
of the engine-turned case of a watch.
[Illustration: Fig. 38.--A small and perfect specimen of
_Stromatopora rugosa_, of the natural size, from the Trenton
Limestone of Canada. (After Billings.)]
Passing next to the sub-kingdom of _Coelenterate_ animals (Zoophytes,
Corals, &c.), we find that this great group, almost or wholly
absent in the Cambrian, is represented in Lower Silurian deposits
by a great number of forms belonging on the one hand to the true
Corals, and en the other hand to the singular family of the
_Graptolites_. If we except certain plant-like fossils which
probably belong rather to the Sertularians or the Polyzoans (e.g.,
_Dictyonema, Dendrograptus_, &c.), the family of the _Graptolites_
may be regarded as exclusively Silurian in its distribution. Not
only is this the case, but it attained its maximum development
almost upon its first appearance, in the Arenig Rocks; and whilst
represented by a great variety of types in the Lower Silurian;
it only exists in the Upper Silurian in a much diminished form.
Pages:
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177