Trilobita without thoracic segments. Cephala and pygidia simple.
Family Naraoidæ Walcott.
Cephalon and pygidium large, both shields nearly smooth. Eyes absent. A single species: Naraoia compacta Walcott, Middle Cambrian, British Columbia.
Subclass Haplopoda nov.
Crustacea with trilobate form, two pairs of uniramous antennæ, no facial sutures, sessile compound eyes present or absent, pygidium and pleural lobes generally reduced, large labrum present, appendages of the trunk biramous.
Order Marrellina nov.
Form trilobite-like, pleural lobes reduced, endobases absent from coxopodites of body, pygidium a small plate.
Family Marrellidæ Walcott.
Cephalon with long genal and nuchal spines. Eyes marginal. A single species: Marrella splendens Walcott, Middle Cambrian, British Columbia.