A new cladid inadunate crinoid, Illemocrinus amphiatus n. gen., n. sp., is described from the Middle Ordovician Bobcaygeon Formation of Ontario. Illemocrinus, the oldest known member of the Thalamocrinidae, is characterized by an obconical cup containing two anal plates including quadrangular radianal, narrow radial facets, isotomously branching arms, and an erect anal tube composed of vertical rows of nodose plates. Illemocrinus probably evolved from Palaeocrinus, indicating a North American origin for the Thalamocrinidae.

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