Abstract
Plethopeltis armatus, and species of Leiocorpyphe, are shown to have in common a median ventral cephalic suture and thoracic axial rings with a poorly-defined articulating furrow and a strong transverse ventral ridge. In other characters they differ greatly, and to group them in the Plethopeltidae is open to question; this family may be polyphyletic. The median ventral suture is regarded as a cardinal character of Asaphida, but it is uncertain whether this character had a single origin or arose more than once. Leiocoryphe gemma may be a paedomorphic species derived from some Upper Cambrian trilobite with an effaced cephalon.
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