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, 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.  They differ from the Crinoids, however, in the fact that the upper surface of the body does not support the crown of branched feathery “arms,” which are so characteristic of the latter.  On the contrary, the summit of the cup is closed up in the fashion of a flower-bud, whence the technical name of Blastoidea applied to the group (Gr. blastos, a bud; eidos, form).  From the top of the cup radiate five broad, transversely-striated areas (fig. 118, C), each with a longitudinal groove down its middle; and along each side of each of these grooves there seems to have been attached a row of short jointed calcareous filaments or “pinnules.”

[Illustration:  Fig. 119.—­Paloechinus ellipticus, one of the Carboniferous Sea-urchins.  The left-hand figure shows one of the “ambulacral areas” enlarged, exhibiting the perforated plates.  The right-land figure exhibits a single plate from one of the “inter-ambulacral areas.” (After M’Coy.)]