ECHINODERMATA. CRINOIDEA. 91 



Antedon, Actinometra), this has a wide range in space and 

 depth, the genus Antedon being found from the shore 

 down to 2900 fathoms. The stalked crinoids are much 

 less abundant than the Comatulidae, and are found mainly 

 at great depths. In most cases the species of crinoids 

 have only a limited distribution in space. 



The Palaeocrinoids are found only in the Palaeozoic 

 formations, the Neocrinoids only in the Neozoic and at 

 the present day. In the Palaeozoic the crinoids are much 

 more numerous than the other Echinoderms, their remains 

 in some cases forming the main part of some limestone 

 beds (crinoidal limestone), as for instance in the Carboni- 

 ferous. None of the other classes are ever sufficiently 

 abundant to be of importance as rock-builders. 



The crinoids occur first in the Tremadoc Beds ; in the 

 Upper Ordovician Glyptocrinus and a few others have 

 been found. In the Silurian they become very much 

 more abundant, attaining their maximum development, 

 the most important genera being Gyathocrinus, Ichthyo- 

 crinus, and Crotalocrinus. In the Devonian, Cyathocrinus, 

 Actinocrinus and others are common ; in the Carboniferous, 

 Actinocrinus, Poteriocrinus, and Platycrinus. Crinoids 

 are rare in the Permian. In the Trias we meet with a 

 new phase in their development, the Palaeocrinoids being 

 replaced by the Neocrinoids ; the characteristic form is 

 Encrinus. In the Jurassic we get Antedon, Pentacrinus, 

 Extracrinus, Apiocrinus, and Millericrinus, the first two 

 living on to the present day. In the Cretaceous the chief 

 forms are Marsupites and Bourgueticrinus. In the Caino- 

 zoic, crinoids are very rare. 



