The definition and scope of Ordovician genera of agnostid trilobites a
re discussed, and the generic affiliation of Ordovician species and we
ll-illustrated material referred to in open nomenclature is assessed.
Some 198 Ordovician species plus 25 species of uncertain latest Cambri
an/earliest Ordovician age are enumerated, representing 24 genera and
subgenera, of which Geragnostus (Novoagnostus) n.subg. and Lotagnostus
(Semagnostus) n.subg. are new. Geragnostella is ranked as a subgenus
of Geragnostus. The species of Arthrorhnchis are tentatively divided a
mong a tardus species group and an elspethi species group. Species of
Dividuagnostus are divided among a sensu stricto group and an early sp
ecies group. Pseudorhaptagnostus Lermontova, 1951 is restored as senio
r synonym of Neoagnostus Kobayashi, 1955. It is concluded that Ordovic
ian agnostids were largely adapted to temperate and colder water habit
ats.