The Middle Ordovician Simpson Group of southern Oklahoma consists of an exceptionally continuous record of carbonates, shales and quartz sandstones which accumulated in a shallow shelf environment. Vertical changes in the distribution of trilobite genera from these rocks can reasonably be interpreted as due to age rather than the influence of various environments or faunal provinces. The affinities of the lower Simpson trilobite faunas are with inner shelf faunas of trilobite zones M and N known from Utah and eastern Nevada and, to a lesser degree, with early Middle Ordovician faunas of Quebec and Newfoundland. Middle Simpson (McLish and Tulip Creek) Formations reflect slightly higher energy conditions and very few well-preserved trilobites. Those few genera present (for example, Pliomerops and Eorobergia) are more typical of the Chazy Group and correlatives from eastern North America. The higher Bromide Formation, also of shelf origin, displays a distinctly eastern fauna, very similar to elements found in the Chazy Group and the presumably younger Edinburg Group of the east. Although many of Cooper's (1956) Middle Ordovician correlations can be understood, the stage names proposed at that time overlap in some cases and lack proper biostratigraphic definition in others. Abandonment of these stage names is urged until description of Middle Ordovician conodont, graptolite and shelly faunas now underway is nearer completion.

Cryptolithinid trilobites have been repeatedly described from North America for over 150 years. Earlier work on these forms is here integrated with new information from Oklahoma to form a strong case for the following conclusions: 1) the genusCryptolithusmigrated to (present-day) eastern North America from Europe at a single time in the early Caradoc; 2) it gave rise rapidly to a western North American relative,Cryptolithoides; 3) both genera were restricted to relatively cool-water shelf environments roughly 30 m deep and remained, in general, geographically isolated for several million years. The Viola Group, geographically between these eastern and western occurrences and representing much of Middle and Upper Ordovician time, chronicles the interplay between these two genera over some six million years.In the biogeographic boundary region formed by Oklahoma, the two supposedly distinct genera showed suprising convergence of fringe pit and other character traits as marine transgression reduced provinciality through Ordovician time. This suprising convergence of genera is interpreted as hybridization of mere subspecies after several million years of incomplete geographic separation.

The Copenhagen Formation of central Nevada is composed of a basal sandstone (member A) and two calcareous siltstone and silty limestone members (members B and C). In areas north and south of Antelope Valley, member B of the formation is recognized, but the overlying member C is not recognized. To the east and southeast the supposedly older uppermost beds of the Antelope Valley Limestone are actually equivalent to the lower beds of member B. To the west in two thrust sequences of the Toquima Range, the Caesar Canyon Limestone of Kay and Crawford and an unnamed limestone are probably equivalent to the uppermost Copenhagen Formation. Species of 21 genera of trilobites are described from members B and C. Four additional taxa have not been identified below familial rank. The most evident change in trilobite assemblages takes place about 100 feet below the top of member C in the type locality around Antelope Valley; these highest trilobites suggest correlation with the Viola Limestone of Oklahoma. The trilobites from lower in member C indicate a Wilderness-Barneveld age.

SummaryThe generic composition of trilobite faunas around the perimeter of North America during the Medial Ordovician is related to the regional palaeogeography deduced from field evidence. Major divisions into shelf, shelf-edge (biohermal mounds) and slope (upper and lower) can be recognized. The Caradoc immersion of the N American plate resulted in landward invasion of pre-adapted slope forms. Some of these genera had European origins and their appearance is due to migration in deeper water facies accompanying closure of the Proto-Atlantic ocean. There is no obvious repeat of Cambrian biomere patterns in the Ordovician although some later Ordovician trilobites, especially shelf inhabitants, had their origins in earlier Ordovician biohermal or slope communities. The origin and composition of the Remopleuridid faunal Province is briefly discussed.