Distinguishing among members of the Hexapodidae Miers, 1886, the Asthenognathinae Stimpson, 1858, and the Chasmocarcininae Serène, 1964, is difficult in fossil specimens that lack preserved pereiopods or sterna. Members of the Hexapodidae are easily identified if sterna and pereiopods are present, because possession of four pairs of pereiopods and seven exposed sternites is diagnostic for the family. Several features of the dorsal carapace are useful in differentiating among genera assigned to that family and other subfamilies belonging to the Goneplacidae and Pinnotheridae; however, dorsal carapace characters are less useful in assigning taxa at the family or subfamily level. A key to the taxa discussed herein permits generic differentiation based upon the relative dimensions of the carapace, the shape and size of the orbits, the shape and size of the rostrum, carapace ornamentation, and the degree of fusion of abdominal somites in males.
A new species of PalaeopinnixaVia, 1966, Palaeopinnixa rotundus, is described from the Eocene Coaledo Formation of Oregon, USA. A new hexapodid genus and species, Globihexapus paxillus, is recognized from the Miocene Astoria Formation of Washington, USA. Prepaeduma decapodaMorris and Collins, 1991, is referred to Hexapusde Haan, 1833. Viapinnixa new genus has been erected to accommodate material previously assigned to Pinnixa (Palaeopinnixa) nodosaCollins and Rasmussen, 1992, and Orthakrolophos has been named to accommodate three species of PalaeograpsusBittner, 1875. Asthenognathus urretae new species is described from the middle Eocene Centinela Formation of southern Argentina.