tones of the Permian), the
great group of the _Rugose corals_, which flourished so largely
throughout the Silurian, Devonian, and Carboniferous periods,
is found to have all but disappeared, and it is never again
represented save sporadically and by isolated forms.

[Footnote 19: A singular fossil has been described by Professor
Martin Duncan and Mr Jenkins from the Carboniferous rocks under
the name of _Paloeocoryne_, and has been referred to the Hydroid
Zoophytes (_Corynida_). Doubt, however, has been thrown by other
observers on the correctness of this reference.]

[Illustration: Fig. 117.--_Platycrinus tricontadactylus_, Lower
Carboniferous. The left-hand figure shows the calyx, arms, and
upper part of the stem; and the figure next this shows the surface
of one of the joints of the column. The right-hand figure shows
the proboscis. (After M'Coy.)]

[Illustration: Fig. 118.--A, _Pentremites pyriformis_, side-view
of the body ("calyx"); B, The same viewed from below, showing the
arrangement of the plates; C, Body of _Pentremites conoideus_,
viewed from above. Carboniferous.]

Amongst the _Echinoderms_, by far the most important forms are
the Sea-lilies and the Sea-urchins--the former from their great
abundance, and the latter from their singular structure; but the
little group of the "Pentremites" also requires to be noticed.
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, a