ECHINODERMA. CRINOIDEA 145 



Distribution of the Crinoidea 



In comparison with the large number of genera which 

 existed in past ages, especially in the Palaeozoic period, 

 the Crinoidea are but poorly represented at the present 

 day. The most important families are the Antedonidse 

 and the Actinometridae ; these are widely distributed, and 

 occur chiefly in shallow water, but some are found at con- 

 siderable depths — Antedon extending from the shore-line 

 down to 2900 fathoms, and Actinometra down to 800 

 fathoms. The stalked crinoids (e.g. Isocrinus, Rhizocrinus) 

 are much less abundant than the Antedonidse and Actino- 

 metridae, and are found mainly at great depths. In most 

 cases the species of crinoids have only a limited distribu- 

 tion in space. 



In the Palaeozoic formations the crinoids are much more 

 numerous than the other Echinoderms, their remains 

 (chiefly stems) forming the main part of some limestone 

 beds (crinoidal limestone or marble), as for instance in the 

 Carboniferous. The other Echinoderms are seldom suffi- 

 ciently numerous to be of importance as rock-builders. 

 The majority of fossil crinoids appear to have lived in fairly 

 shallow water, since they are found in association with 

 reef-building corals and other shallow-water organisms. 



Crinoids occur first in the Tremadoc Beds. In the Ordo- 

 vician, Glyptocrinus, Dendrocrinus, and a few others have 

 been found. In the Silurian, crinoids become very much 

 more abundant, and attain their maximum development ; 

 the most important genera are Botryocrinus, Calceocrinus, 

 Or otalo crimes, Eucalyptocrinus, Gissocrinus, Ichthyocrinus, 

 Marsipocrinus, Periechocrinus, Pisocrinus, Sagenocrinus, 

 Taxoorinus. In the Devonian, Cyathocrinus, Cupresso- 

 crinus, Haplocrinus, Hexacrinus and others are common ; 

 w. p. 10 



