512 A POPULAR EXPOSITION OF THE 
1. Crinoida.—In the majority.of fossil crinoids or encrinites (*sea- 
lilies ’’), the general form consists of a body or digestive sack, covered 
by calcareous plates, and furnished at its upper part with a series of 
jointed arms or tentacles, and at its lower part with a jointed and per- 
forated stem (composed of numerous round or pentagonal plates) by 
which it was attached to the sea-bottom: see fig. 82 This Order is 
of great paleeontological interest. In the 
scas of the Paleeozoic and Mesozoic periods, 
its representatives swarmed im vast numbers; 
whilst but few forms belonging to it have 
been obtained from Tertiary rocks (see the 
Table of Formations on page 453 above) ; 
and in existing seas the order is almost 
extinct, two or three species alone remain- 
ing to represent it. The best known of 
these is the Pentacrinus caput-Meduse of 
the West Indian seas. A small species of 
Comatula exists also in the Irish Channel, 
and of late years has been carefully studied. 
This form is fixed by a stem in the early 
condition, andafterwards becomes free. The Fig. 82. 
fixed stage was originally thought to be permanent, and the species 
was known as Pentacrinus Europeus. The genus Marsupites, of the 
Cretaceous rocks, was also a free form, during a portion, if not during 
the entire period, of its life. 
The cup-shaped body of the crinoid animal is technically termed 
the “calyx.” It is enclosed by numerous polygonal plates, arranged, 
for each genus, in definite order. The plates in a row immediately 
above the stem are commonly knownas “ basals.” These are usually 
three or five in number. The next series, absent, however, in many 
genera, are called sub-radials, and the next, supporting the base of the 
arms, are known as “radials.” The radials always range in five 
vertical rows, each row being made up of one or several plates, 
between which occur other plates, termed inter-radials and anal plates. 
The upper part of the calyx is covered (in most genera) by numer-. 
ous small and irregular plates, termed, collectively, the “vault.” 
The vault-plates are sometimes prolonged into a so-called “ trunk,” 
the office of which is still undetermined* In some species the vault 
has two openings, in others only one. 
