The receptacle being round and inflated, and composed of pieces articulated with the stems, and supporting the arms by similar articulations, are the generic characters of Apiocrinites of Miller. When round but not inflated—Encrinus, when pentagonal, Pentacrinus. When the receptacle is composed of angular plates united at the edges and forming several series or stages, it constitutes the basis of the following genera: viz,—
Platycrinus; two series, the one of three, the other of five plates.
Pateriocrinus; three series each of five plates.
Cyathocrinus; three series of five plates, the last with five intercalated pieces.
Actinocrinus; several series of plates: the first composed of three, the second of five, and the others of many pieces.
Rhodocrinus; several series of plates that are covered externally with radiated ridges. The first course consists of three, the second of five, the third of ten, and the others of many plates.
Eugeniacrinus; of five pieces united into one receptacle.
This brief explanation will enable the student to comprehend the nature of the almost infinite variety of figure and ornament which the fossil crinoidea present, and the principles of nomenclature generally adopted by modern authors. To attempt an enumeration even of the genera would be inadmissible in these volumes. The late Mr. Miller's "Natural History of the Crinoidea or Lily-shaped Animals,"[271] will afford the student satisfactory information on this class of organic remains.[272]
[271] Published in 1 vol. 4to. 1821, with coloured plates.
[272] A beautiful "Monograph on the Recent and Fossil Crinoidea," by Messrs. Austin, in 4to. is in course of publication, of which but eight numbers have appeared.