An early Ordovician patelliform gastropod, Palaelophacmaea , reinterpreted as a coelenterate

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Abstract
The fossil P. criola Donaldson, from the Early Ordovician Stonehenge Formation of central Pennsylvania [USA], was described as a patelliform gastropod. A reinterpretation of the type lot and study of a few additional specimens provide the basis for an alternative placement. Palaelophacmaea is assigned to the Hydrozoa, as a possible chondrophore. It has an exceptionally thin shell or test and concentric but irregular corrugations. Cambrian univalve genera having a more or less circular outline that are currently assigned to the Gastropoda or Monoplacophora should be reexamined to see whether they have features of fossil chondrophore coelenterates rather than those of mollusks. The Late Cambrian Palaeoacmaea Hall et Whitfield is removed from the monoplacophoran Mollusca and left unassigned as to phylum. At least some Early Cambrian species of Scenella are probably coelenterate remains.