ECHINODEEMA — CRINOIDS. 
63 
which iiiiiny magniticeut examples from Lyme llegis and 
elsewhere are exhibited. Here we note how colonies were 
Ibrmed of many individuals of only one or two species, as is 
the case to-day. A portion of such a colony from the Lias 
of Boll, in Whrtemberg, forms a beautiful picture in the 
middle of the case (Plate IV). The stem of this form is 
.said to reach a length oJ' 50 feet; a length of 15 feet is 
certainly common. The length of stem is perhaps to be 
explained by the fact that many of these Liassic Pentacrini 
were attached to floating pieces of wood, and so hung crown 
downwards. A closely allied form is Isocrinus, which gi’ows 
in forests on the floor of some recent seas. Various species 
will be found among the Triassic, Jurassic, Cretaceous, and 
Tertiary crinoids. The elegant five-petalled stem-segments 
of both these genera are washed out of the rocks in many 
places, and to them the name Pentacrimis (five-lily) was first 
given by Agidcola in 1546. The Pear Encrinite (Apiocrinus) 
from the Bradford Clay of Wiltshire has the top part of its 
stem greatly thickened. Near it is Millerio'inus Pratti, 
which exemplifies the tendency, constant in crinoids, to 
loosen their attachment to the sea-floor and to become 
free-moving, witli a shortened stem. The stem is reduced 
to a mere knob iwAnteilon and Actimmdra, which, bemnnius 
in the Oolites, occur in vast numbers in modern seas. 
Though unstalked and free-moving when grown up, these 
crinoids are fixed by a stalk when cpiite young. 
In the Cretaceous Crinoids, and Uinlacrhi'm, 
the stem is entirely lost, and it seems probable that the latter 
at all events was a free-swimming form. Both genera lived 
at almost the same time (Upper Senonian) and were widely 
distributed. Specimens are shown from the English Chalk, 
and a slab covered with Uintacrinus from North America is 
placed on the wall. 
Tertiary Crinoids are not uumeujus. The most inter- 
esting specimens are tliose illustrating variation in the stem 
of Buhinocrimis, another Pentacrinid. 
Gallery 
VIII. 
Wall-case 
16 . 
Wall-case 
18 . 
Table-case 
31 . 
Table-case 
31 . 
Between 
Wall-cases 
16 & 17 . 
W all-case 
17 . 
Class CYSTIDEA. 
The Cystids are of interest partly on account of their 
rarity, partly because they are all extinct, none having 
survived tlie Carboniferous Epoch, partly l>y reason of their 
diversity and strangeness of structure, but mainly because 
they are thought to comprise forms from which other classes 
