nd 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.)]

A few Star-fishes and Brittle-stars are known to occur in the
Carboniferous rocks; but the only other Echinodemls of this period
which need be noticed are the Sea-urchins (_Echinoids_). Detached
plates and spines of these are far from rare in the Carboniferous
deposits; but anything like perfect specimens are exceedingly
scarce. The Carboniferous Sea-urchins agree with those of the
present day in having the body enclosed in a shell formed by
an enormous number of calcareous plates articulated together.
The shell may be regarded as, typically, nearly spherical in
shape, with the mouth in the centre of the base, and the excretory
opening or vent at its summit. In both the ancient forms and the
recent ones, the plates of the shell are arranged in ten zones
which generally radiate from the summit to the centre of the base.
In five of these zones--termed the "ambulacral areas"--the plates
are perforated by minute apertures or "pores," through which
the animal can protrude the little water-tubes ("tube-feet") by
which its locomotion is carried on. In the other five zones--the
so-called "inter-ambulacral areas"--the plates are of larger
size, and are not perforated by any apertu