Abstract
Crinoid taxa based solely on parts of the stem lead to potential problems of synonymy and to nomenclatural instability. Many stem morphotypes appear to be vicarious, and homeomorphy may be common among even the most specialized appearing stems. The affinity of the crinoid stem 'genus' Platyplateium Moore and Jeffords to the camerate crinoid family Platycrinitidae is confirmed by its association with a Platycrinites-like calyx (Platycrinites nikondaense n.sp.). Platycrinites nikondaense from the Permian of Alaska and P. ellesmerense n. sp. from the Permian (Guadalupian) of Ellesmere Island represent species that have evolved by means of a secondary decrease in number of tegmen plates and modification of the stem from the Platycrinites type to the platyplateioid form. Platycrinites remotus Strimple and Watkins, from the Upper Carboniferous of Texas represents an earlier stage in tegminal evolution, but had probably already developed a platyplateioid stem.
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