A review of the British Silurian cystoids

New interpretations of morphology, new occurrences and new taxa of British Silurian cystoids are summarized. Two ambulacral patterns occur in the Callocystitidae. In one the first two brachiole facets branch to the left in ambulacra B and D, but only the first one in ambulacra A, C and E (the B + D arrangement). In the alternative pattern all ambulacra are the same with only the first brachiole branching to the left. The B + D pattern is plesiomorphic. Among callocystitids with four ambulacra the type species of Tetracystis has the B + D arrangement as does ‘Apiocystites’ elegans; both are referred to Tetracystis. The British species ‘T.’ oblongus has all ambulacra the same and becomes type species of the new genus Troosticystis. New records include Homocystites brenchleyi sp. nov. from the Rhuddanian of South Wales, which demonstrates that the family Cheirocrinidae did not become extinct at the end of the Ordovician as previously suspected. The British Silurian cystoid Prunocystites fletcheri has been recorded from Norway, the only species known to occur outside Britain. Schizocystis armata (Forbes, 1848) occurs at a new locality near Buildwas, Shropshire. Glansicystis glans sp. nov. is described for ‘the common form’ of G. baccata. It differs from G. baccata in having a larger oral area, weaker thecal ornament, shallower plate sutures and a more cylindrical theca.