Cephalaspids from the Upper Silurian of Oesel (Estonia), with a discussion of Cephalaspid genera

The Cephalaspidae of the middle Ludlow, upper Silurian of Oesel are described. They are Thyestes verrucosus Eichwald, Witaaspis schrenckii (Pander), Witaaspis patteni Robertson, Saaremaaspis mickwitizi (Rohon), and Cephalaspis oeselensis Robertson. Eukeraspis pustulifera Rohon is not identifiable. The basis for familial, generic, and specific categories in the Osteostraci is discussed. Didymaspis is raised to familial rank. A classification of the Osteostraci is given.