The _Protozoans_ are represented principally by _Stromatopora_ and
_Ischadites_, along with a number of undoubted sponges (such as
_Amphispongia, Astroeospongia, Astylospongia_, and _Paloeomanon_).
Amongst the _Coelenterates_, we find the old group of _Graptolites_
now verging on extinction. Individuals still remain numerous,
but the variety of generic and specific types has now become
greatly reduced. All the branching and complex forms of the Arenig,
the twin-Graptolites and _Dicranograpti_ of the Llandeilo, and
the double-celled _Diplograpti_ and _Climacograpti_ of the Bala
group, have now disappeared. In their place we have the singular
_Retiolites_, with its curiously-reticulated skeleton; and several
species of the single-celled genus _Monograptus_, of which a
characteristic species (_M. Priodon_) is here figured. If we
remove from this group the plant-like _Dictyonemoe_, which are
still present, and which survive into the Devonian, no known
species of _Graptolite_ has hitherto been detected in strata
higher in geological position than the Ludlow. This, therefore,
presents us with the first instance we have as yet met with of
the total disappearance and extinction of a great and important
series of organic forms.
[Illustration: Fig. 58.--A, _Monograptus priodon_, slightly enlarged.
B, Fragment of the same viewed from behind.
Pages:
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203