Pterygometopidae are common trilobites in Middle and Upper Ordovician rocks of North America, northern Europe, and Siberia and they have considerable biogeographic usefulness. The subfamilies Pterygometopinae and Chasmopinae characterize Baltica, the Eomonorachinae characterizes Laurentia, and the Monorakinae characterizes Siberia. In North America, pterygometopids occur in strata of Whiterockian to Gamachian (Llanvirn to Ashgill) age. The Pterygometopinae is represented by Achatella Delo, the Monorakinae by Monorakos Schmidt, the Chasmopinae by Toxochasmops McNamara, and the Eomonorachinae by Eomonorachus Delo, Calyptaulax Cooper, Sceptaspis n. gen., Denella n. gen., and Tricopelta n. gen. Isalaux Frederickson and Pollack is not assigned to a subfamily. The north European Pterygometopus Schmidt was probably ancestral to Calyptaulax, which, in turn, gave rise to a modest North American radiation of eomonorachines in the Blackriveran to Edenian interval. The eomonorachines display both sphaeroidal and uncoiled spiral enrollment patterns, as well as a new intermediate pattern. A neotype is chosen for Eomonorachus intermedius (Walcott). Denella cumera and Tricopelta mackenziensis are new.