The Sea-lilies are so abundant in the Carboniferous rocks, that it
has been proposed to call the earlier portion of the period the
"Age of Crinoids." Vast masses of the limestones of the period
are "crinoidal," being more or less extensively composed of the
broken columns, and detached plates and joints of Sea-lilies,
whilst perfect "heads" may be exceedingly rare and difficult
to procure. In North America the remains of Crinoids are even
more abundant at this horizon than in Britain, and the specimens
found seem to be commonly more perfect. The commonest of the
Carboniferous Crinoids belong to the genera _Cyathocrinus,
Actinocrinus, Platycrinus_, (fig. 117), _Poteriocrinus, Zeacrinus_,
and _Forbesiocrinus_. Closely allied to the Crinoids, or forming
a kind of transition between these and the Cystideans, is the
little group of the "Pentremites," or _Blastoids_ (fig. 118).
This group is first known to have commenced its existence in
the Upper Silurian, and it increased considerably in numbers
in the Devonian; but it was in the seas of the Carboniferous
period that it attained its maximum, and no certain representative
of the family has been detected in any later deposits. The
"Pentremites" resemble the Crinoids in having a cup-shaped body
(fig. 118, A) enclosed by closely-fitting calcareous plates,
and supported on a short stem or "column," composed of numerous
calcareous pieces flexibly articulated together.
Pages:
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278