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Rhabdocrinus n.g. from the Scottish Carboniferous Limestone

Published online by Cambridge University Press:  01 May 2009

James Wright
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When describing the Scottish species Poteriocrinus scotocarbonarius, doubts were expressed about the propriety of assigning it to Poteriocrinus (Wright, 1937, p. 402, 1939–40, p. 17). Before creating a new genus, however, it was considered desirable to make an examination of one of the chief figured specimens of the genotype species P. crassus Miller and to compare it with P. plicatus Austin. Up to the present it has not been possible to do this, and such an outcome seems unlikely for some time to come. In view of the great amount of work that has been done on this and allied genera in recent years, it is now reasonably certain that our Scottish forms are quite distinct from any English or American species or genera hitherto described. Hence the new genus Rhabdocrinus now proposed. The name Poteriocrinus is now discarded in favour of the original Poteriocrinites (Bassler, 1938, Moore and Laudon, 1943). The genotype species was described by Miller in 1821. The specimen illustrated by him as fig. 1 on the plate opposite p. 69 is intended to portray a complete cup apparently from the right posterior side. It has a few columnals attached. According to this figure the radials have horseshoe-shaped facets. The Austins state that this figure is a restoration, and they give an illustration of the same specimen from the same position (Austin, T. and T., 1843–7, pi. 8, fig. 3c, p. 74). Here also the radial facets are not well shown. There is accordingly still a slight doubt regarding these structures on this specimen. On Miller's plate, however, are several figures of detached radials and on the plate opposite p. 67 is a diagrammatic representation of the complete cup of P. crassus. All these figures show radials with horseshoe-shaped facets and a distinct articulating cross ridge, the facets themselves only occupying part of the upper surface of the radials. This type of structure is taken as typical of the genus Poteriocrinites. In the new genus now proposed the facets are quite different in shape and occupy nearly all the upper surfaces of the radials with no trace whatsoever of a cross ridge. The pattern of the surfaces in fact consists of a series of fine crenulations or striations which cross the facets in a transverse direction, i.e. from front to back. In addition, some facets are characterized by little pustules and prominences which are more commonly observed near the centre and towards the back or interior surface of the facets. These features constitute the chief distinguishing character of the new genus. As to the family to which Rhabdocrinus should be assigned, it clearly does not belong to the Poteriocrinitidae as now defined (Moore and Laudon, 1943, p. 54, fig. 5), and since it is impossible under prevailing conditions to trace any known family to which it can be referred, the matter is left open at present.

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Articles
Information
Geological Magazine , Volume 81 , Issue 6 , December 1944 , pp. 266 - 271
Copyright
Copyright © Cambridge University Press 1944

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References

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