A new discoid heliolitid (Proporidae) from the Upper Ordovician of the Baltic area: Morphology and modalities of increase

A species of Proporidae (Heliolitida) forming very small, discoid coralla, is fully described. It is mainly preserved in natural moulds in the Öjlemyr chert, an erratic silicified limestone found in Eridanos deposits surfacing in the west and north-east of the Dutch Overijssel-province and adjoining German area, generally accepted to be Uppermost Katian. The material is assigned to the rehabilitated Pinacopora Nicholson & Etheridge, 1878, a genus closely allied to Propora Milne-Edwards & Haime, 1849, and a new species, Pinacopora baltica is named. The main feature of this species, besides the characteristics of the genus and the biometric data, is the presence of wide, short, and convex septal ridges bearing scattered septal spines, alternating with narrow interseptal furrows. For the first time in heliolitids a pleurodictyoid pattern of growth is described. Only two species of Pinacopora are recorded: P. baltica n. sp. (Upper Ordovician) and P. grayi Nicholson & Etheridge, 1878 (Lower Silurian), respectively from Baltica and Laurentia.