Ramseyocrinus and Ristnacrinus from the Ordovician of Britain
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Ramseyocrinus Bates, 1968, has hitherto been included in the family Eustenocrinidae Ulrich, 1925 but it differs from all other members of this family in having only four radials, an anal X which is supported by two radials (rather than two or three superradials, or a brachianal and superradial), a cup which is about as wide as high, and a column which is tetrameric proximally and tetragonal holomeric distally. A new family Ramseyocrinidae, is erected for this genus. Of the true eustenocrinids, Ristnacrinus Opik, 1934 is recognized from the British Ordovician for the first time, based upon the occurrence of its distinctive columnals with synarthrial articulating ridges. Ristnacrinus columnals are known from four localities of Cautleyan-Rawtheyan (?Hirnantian) age and comparison with similar columnals from the Swedish Boda Limestone (Ashgill) suggests that more than one species may be present in Britain.
Palaeontology - Volume 27 Part 3 Pages 623-634Palaeontology - Volume 27 Part 3 Pages 623-634
Citations
DONOVAN, S. K. 1984. Ramseyocrinus and Ristnacrinus from the Ordovician of Britain. Palaeontology, 27, 3, 623–634.