ECHINODERMA — CRINOIDS. 



63 



which many magnificent examples from Lyme Regis and 

 elsewhere are exhibited. Here we note how colonies were 

 formed 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 Wiirtemberg, forms a beautiful picture in the 

 middle of the case (Plate IV). The stem of this form is 

 said to reach a length of 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 grows 

 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 Fentacrinus (five-lily) was first 

 given by Agricola in 1546. The Pear Encrinite (Apiocrinus) 

 from the Bradford Clay of Wiltshire has the top part of its 

 stem greatly thickened. Near it is Millericrimis Pratti, 

 which exemplifies the tendency, constant in crinoids, to 

 loosen their attachment to the sea-floor and to become 

 free-moving, with a shortened stem. The stem is reduced 

 to a mere knob inAntedon and Actinometra, which, beginning 

 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 quite young. 



In the Cretaceous Cvinoidi^j Marsiqntes and Uintacrinus, 

 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 numerous. The most inter- 

 esting specimens are those illustrating variation in the stem 

 of Balanocrinus, another Pentacrinid. 



G-allery 

 VIII. 

 Wall-case 

 16. 



Wall-case 

 18. 



Table-ease 

 31. 



Table-case 

 31. 



Between 

 Wall-cases 

 16 & 17. 



Wall-case 

 17. 



Class CYSTIDEA. 



The Cystids are of interest partly on account of their 

 rarity, partly because they are all extinct, none having 

 survived the Carboniferous Epoch, partly by reason of their 

 diversity and strangeness of structure, but mainly because 

 they are thought to comprise forms from which other classes 



