486 CRINOIDS IN PRIMARY STRATA. 



or calyx, surrounded by well-developed arms, which recall the arms 

 of a star-fish, or the branching arms of Euryalus. Some types have the 

 calyx covered with plates, and then the base of the calyx is commonly 

 formed by five plates; the genera with a single basal plate usually have 

 the calyx open above (fig. 99^). Crinoids are numerous in the Primary 

 rocks, and less common in the Secondary strata. They are now verging 

 on extinction, though several genera still survive, such as Pentacrinus, 

 Conocrinus, Bathycrinus, Hyocrinus, and Holopus. 



The oldest known genus is Dendrocrinus from the Tremadoc 

 rocks. The Llandeilo beds yield Glt/ptocrinus, Cyathocrinus, JKJiodo- 

 crinus, and Adinocrinus, the last three surviving till the Carboniferous. 

 In America Crinoids are more numerous in the Cambrian period, and 

 comprise Hybocrinus, Anomalocrinus, Homocrinus, Cupalocrinus. 



The Wenlock period is remarkably rich in Crinoids, especially in 

 the families represented by Cyathocrinus, which ranges to the Per- 

 mian ; TaxocrinuSj Iclithyocrinus, Cheirocrimcs, Habrocrinus, and Pote- 



Fig. 94. Cyathocrinus. Fig. 95. Taxocrinus. 



riocrinus, which range to the Carboniferous, and such peculiar genera 

 us Crotalocrinus, Carpocrinus, Briarocrinus, Dimerocrinus, Periecho- 

 crinus, Corymlocrinus, and Pisocrinus. Several genera, like Euca- 

 lyptocrinuS) survive to the Devonian. The Devonian Crinoids are 

 also numerous. They comprise Cupressocrinus, Symbathocrinus, RMpa- 

 docrimts, Grasterocoma, Culicocrinus, Dolatocrinus. In the Carbon- 

 iferous limestone Crinoids are particularly abundant, sometimes 

 forming the entire mass of the rock. A large number of species be- 

 long to such genera as Poteriocrinus, Platycrinus, and Actinocmnus, or 

 to sub-genera grouped under these types. Other genera characteristic 

 of this period are Woodocrinus, Agassizocrinus, Dichocrinus, Ollo- 

 crinus, Graphiocrinus, Onychocrinus. In the Dyas, or Permian, the 

 only Crinoid is Cyathocrinus, already mentioned. 



The Trias is characterised by Encrinus, which belongs to the arti- 

 culate sub-order, in which all the secondary Crinoids find a place. 

 In this formation Pentacrinus appears for the first time, though it 

 attains its most remarkable development in the Lias. Cotylederma is 

 Liassic. 



The Jurassic genera comprise Eugeniocrinus, Triacrinus, Phyllo- 



