A review of Ordovician agnostid genera (Trilobita)

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Abstract
The definition and scope of Ordovician genera of agnostid trilobites are discussed, and the generic affiliation of Ordovician species and well-illustrated material referred to in open nomenclature is assessed. Some 198 Ordovician species plus 25 species of uncertain latest Cambrian/earliest Ordovician age are enumerated, representing 24 genera and subgenera, of whichGeragnostus (Novoagnostus)n.subg. andLotagnostus (Semagnostus)n.subg. are new.Geragnostellais ranked as a subgenus ofGeragnostus. The species ofArthrorhachisare tentatively divided among atardusspecies group and anelspethispecies group. Species ofDividuagnostusare divided among asensu strictogroup and an early species group.PseudorhaptagnostusLermontova, 1951 is restored as senior synonym ofNeoagnostusKobayashi, 1955. It is concluded that Ordovician agnostids were largely adapted to temperate and colder water habitats.