Ordovician Mohawkian Ostracoda: Lower Trenton Decorah Fauna

The Decorah formation, composed principally of shales in the northern part of the outcrop in Iowa, Minnesota, and Western Wisconsin, has an ostracode fauna of about 80 species (28 new), 70 of which have been found in recently collected faunules; there are representatives of 39 genera, many of which have been based on Decorah species. A revised classification of the families Primitiidae and Beyrichiidae is presented. The new genera Saccelatia, Winchellatia, Maratia, Parenthatia, and Opikatia are defined. The new genus Bolbiprimitia is proposed for the Silurian Halliella fissurella Ulrich and Bassler. Dimorphism is represented in 12 of the genera, evidenced by marginal and intramarginal structures present in some forms and lacking in otherwise similar specimens. The two members of the formation- the Guttenberg and the Ion-have some distinctive species, but most of the Decorah forms are represented in both members. Faunas from the northern and southern parts of the belt of outcrop differ distinctly. The Decorah ostracode fauna is closely similar to that in the lower Trenton of Ontario; no other faunas of com- parable size and age have been described in North America. The faunas of the Uhaku and Kukruse formations of the Baltic region are probably of similar age, but have no identical species and few that are similar to those in the Decorah. The interior North American and northern European faunas are of different faunal provinces; the Decorah fauna must be indigenous or of southern origin; many Upper Ordovician ostracodes of the interior of North America are quite similar to the Decorah species, and they would seem to be descendants