Baumiller, Tomasz K. Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan.
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One of three Paleozoic subclasses of the Crinoidea consisting of over 350 genera ranging from the Ordovician to the Permian. Its members generally lack fixed brachials such that the arms are free above the level of the radials. The calyx may be monocyclic, consisting of a circlet of basal and a circlet of radial plates (illus. a), or dicyclic, having an additional circlet of infrabasal plates (illus. b). The arms, constructed of serially arranged brachials, may be simple or branched. Each arm plate may be bare or give rise to a slender, unbifurcated branchlet called a pinnule. Articulating surfaces of adjacent brachials may be smooth, forming a tight, inflexible suture, or each may possess a central ridge. In the latter case, abutting ridges of adjacent brachials form a mechanical fulcrum allowing a substantial degree of movement and flexibility. The anus may lie flush with the tegmen or be elevated on a tube of variable length and diameter.
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