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A silicified ostracode fauna consisting of 54 species, 44 of them new, from the Middle Ordovician (Chazyan-Trentonian) Lincolnshire and Edinburg formations of the Shenandoah Valley of Virginia is described. Among the genera studied are: A parchites, Oepikella, Leperditella, Conchoprimitia, Schmidtella, Milleratia, Hallatia, Euprimitia, Ectoprimitia?, Primitiella, Chilobolbina, A patochilina, Eurychilina, Ctenobolbina, Aechmina?, Monoceratella, Winchellatia?, Thomasatia, Balticella, Bromidella, Bassleratia, Kirkbya?, Krausella, Macrocyproides, Camdenidea?, Bairdiacypris, Bairdianella?, Budnianella, Platyrhomboides, and Bythocypris?. Three new genera—Eokloedenella, Eographiodactylus, and Shenandoia—are proposed. A new family, the Budnianellidae, is proposed to include the genera Budnianella and Platyrhomboides, which bear no close affinities to any previously described family.

Fourteen of the species occur in other formations. Nine of these are in the Chazyan Crown Point formation on Valcour Island, Lake Champlain. Comparative species are also in the Bromide formation (Black Riverian and Chazyan) of the Arbuckle Mountains, formations of the Trentonian and Richmondian stages of north-central United States, the Middle Ordovician of Arctic Canada and North Greenland, and the Upper Ordovician of North Europe.

Preliminary considerations are given to the phylogeny of the fauna. Leperditella is probably the root stock from which the Euprimitiidae developed through Schmidtella, Eridoconcha, Cryptophyllus, and Milleratia. Close relationships are pointed out between the Primitiidae and the Eurychilinidae. The writer suggests that the Eurychilinidae, Oepikellidae, and Aechminidae descended from Aparchites. Primitiella and closely related genera are associated with Eurychilina but may very likely have developed independently. Relations between Ctenobolbina and Winchellatia and primitive sigmoopsids are discussed. The closely related Bassleratia and Thomasatia probably developed from primitive tetradellids via Steusloffia. Two early kloedenellids, Eokloedenella and Balticella, are suggested as the root stock of that group. Eographiodactylus, a new genus, is proposed as a primitive form from which Graphiadactyllis developed. Bairdiacypris, Shenandoia, early Paleozoic “Bythocypris”, and Macrocyproides are descended from a common unknown ancestor. Bairdianella? is suggested as the root stock of the bairdiids, closely related to Camdenidea. Krausella and Rayella are very closely related if not synonymous.

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