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.
Silurian and Upper Ordovician Atrypids of the genera Plectatrypa and Spirigerina