In the
annexed illustration (fig. 64) some of the characteristic Upper
Silurian Trilobites are represented--all, however, belonging
to genera which have their commencement in the Lower Silurian
period. In addition to the above, the Ludlow rocks of Britain
and the Lower Helderberg beds of North America have yielded the
remains of certain singular Crustaceans belonging to the extinct
order of the _Eurypterida_. Some of these wonderful forms are
not remarkable for their size; but others, such as _Pterygotus
Anglicus_ (fig. 65), attain a length of six feet or more, and
may fairly be considered as the giants of their class. The
Eurypterids are most nearly allied to the existing King-crabs
(_Limuli_), and have the anterior end of the body covered with
a great head-shield, carrying two pairs of eyes, the one simple
and the other compound. The feelers are converted into pincers,
whilst the last pair of limbs have their bases covered with spiny
teeth so as to act as jaws, and are flattened and widened out
towards their extremities so as to officiate as swimming-paddles.
The hinder extremity of the body is composed of thirteen rings,
which have no legs attached to them; and the last segment of
the tail is either a flattened plate or a narrow, sword-shaped
spine. Fragments of the skeleton are easily recognised by the
peculiar scale-like markings with which the surface is adorned,
and which look not at all unlike the scales of a fish.
Pages:
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209