Silurian and Upper Ordovician Atrypids of the genera Plectatrypa and Spirigerina

The nomenclatural history of Plectatrypa and Spirigerina is traced and the basis for recognition of each taxon as a valid genus is developed. The earliest progenitors of Spirigerina marginalis, which range from the Upper Ordovician into the Lower Llandovery, are separated as a new subgenus Spirigerina (Eospirigerina), based on Atrypa praemarginalis Savage which is recognized in mid-North America and in Gaspe. The closely related S. (Eospirigerina) gaspeensis is identified in Britain and Venezuela in the Lower Llandovery. True Plectatrypa is unknown below the Lower Llandovery; previous citations from the Ordovician are of forms that the writers propose to include in Eospirigerina. A new species, Plectatrypa henningsmoeni, is proposed for a Lower Llandovery form from the Oslo region. Plectatrypa is regarded as an early offshoot of the Atrypinae, off the main line of evolution that resulted in Atrypa in a strict sense and then little differentiated from Eospirigerina, the earliest known member of the Carinatininae. The species and distribution of Plectatrypa and of Spirigerina are listed.