A revision of Palaeacmaea (Upper Cambrian) (?Cnidaria)

Palaeacmaea typica, the type species of the genus, and P. irvingi, the only other Late Cambrian taxon considered congeneric are redescribed herein. Th eir morphology suggests that they are neither Monoplacophora, where they ar e currently assigned, nor are they Mollusca. Specimens of P. irvingi demons trate considerable variation in shape, interpreted as distortion of a flexi ble integument, of essentially no thickness. A neotype is designated for P. irvingi; it is interpreted as a medusiform fossil, possibly a enidarian. O nly the holotype of P. typica is known, but it shows comparable features. T he genus and the family Palaeacmaeide are placed in Phylum Incertae Sedis. Four Ordovician species previously assigned to Palaeacmaea, show none of th e characteristics noted, and they are tentatively reassigned to other gener a.