Shelf-derived limestone boulders in debris flows of the Cow Head Group, western Newfoundland, have yielded rich mid- to Upper Cambrian trilobite faunas. The agnostoid component of Marjuman boulders consists of 23 species assigned to 11 genera; Kormagnostus boltoni and Kormagnostus? copelandi are new. Cladistic parsimony analysis of the Ptychagnostidae Kobayashi reduces this family to 3 genera: Ptychagnostus Jaekel, Pseudophalacroma Pokrovskaya and Lejopyge Hawle and Corda; Ptychagnostus includes three non-obligate subgenera: P. (Ptychagnostus) (= Tomagnostus Howell); P. (Goniagnostus) Howell and P. (Aotagnostus) öpik (= Myrmecomimus öpik). Correlation with zonations established in northern Canada and Utah indicates that the Cow Head Group includes a nearly complete sequence of agnostoid faunas from the mid-Cambrian Ptychagnostus gibbus Zone to the Upper Marjuman Cedaria brevifrons Zone.