Echinoderms from the Ordovician of the Girvan district, Scotland, with the description of two new species

Two new species of Ordovician echinoderms are described: Pleurocystites mitratus from the Kiln Mudstones (Caradoc) at Craighead, and Homocystites beggi from the Starfish Bed (Ashgill) of Threave Glen. Description of the former necessitated a review of the composition and status of the family Pleurocystitidae. The genus Dipleurocystis Jaekel 1918 and the subgenera Pleurocystites s.s. and Pleurocystella Paul, 1967 are rejected. The number and type of pectinirhombs; size of the periproct, composition of its border and nature of its covering plates (periproctals), and the ornament of the thecal plates and proximal columnals, are the most useful taxonomic criteria within the Pleurocystitidae. Pleurocystites mitratus is characterized by three conjunct pectinirhombs, a symmetrical theca without a rectal lobe and small periproctals each with a central pit. Homocystites beggi is distinguished from the closely related H. constrictus by strong granular ornament on the thecal plates.

Other echinoderm remains include plates of a cheirocrinid cystoid from the Kiln Mudstones.