Abstract: A classification of all genera of Triassic Ammonoidea plus Permian
     Ceratitida is introduced.  Three orders are represented: Prolecanitida (3 genera),
     Ceratitida (443) and Phylloceratida (14).  It is generally agreed that Prolecanitida
     gave rise to the Ceratitida in the Permian; Ceratitida to the Phylloceratida in the
     Lower Triassic.  The Triassic Prolecanitida are interpreted to comprise only
     Episageceratidae.  Sagecerataceae, generally classified as Prolecanitida, are here
     interpreted as Ceratitida.  Xenodiscidae are interpreted as the most important
     group, from a phylogenetic standpoint, to bridge the Permian-Triassic boundary,
     being the source of most if not all other Triassic Ceratitida and probably also the
     Phylloceratida.  Medlicottiaceae (Episagoceratidae) and Otocerataceae also cross
     the boundary but were phylogenetically unimportant although it is possible that
     Otocerataceae gave rise to Sagecerataceae.  Triassic precursors of the Jurassic
     amonites classed as Ammonitina and Lytoccratina in the Treatise (Arkell et al.,
     1957) have not been definitely identified, Phyllocerataceae being the only group
     common to the Triassic and Jurassic and the only group certainly known to
     bridge the Triassic-Jurassic boundary.