A new Upper Ordovician aristocystitid diploporite genus (Echinodermata) from the Llanddowror district, South Wales

Binocalix dichotomus gen. et sp. nov. is the first British Ordovician aristocystitid diploporite to be named. It is known from a single holotype that is partly disarticulated and incomplete. Nevertheless, it differs from all previously described aristocystitids in having a pair of relatively large ambulacral facets developed above a smooth ambulacral platform and on a single circum-oral plate. When densely developed, the diplopores become polygonal and have a central tubercle. They closely resemble those of the sphaeronitid subgenus Sphaeronites (Peritaphros), but all species of Sphaeronites s.l. have narrow ambulacral grooves ending in small facets. The similarities in the diplopores are thought to be convergent. Among aristocystitids, only "Calix" inornatus and Triamara are known to have paired ambulacral facets developed on single circum-oral plates. The former differs from Binocalix in having a smooth thecal surface and flush ambulacral facets. Triamara has raised ambulacral facets but lacks the distinctive ambulacral platforms found in Binocalix. Its diplopores always lack central tubercles, and currently it is only known from the Silurian.