Subgenera. According to Bernard Joly (1993), the specialist in Phyllocerataceae, the genus Phylloceras comprises two subgenera. Hypophylloceras Salfeld, 1924 (type Phylloceras onoense Stanton, 1895) includes more or less compressed forms with complex, deeply incised sutures and tetraphyllous saddles (i.e., ending in four phyllites) or saddles tending towards tetraphyllous. It is known from the Valanginian to the Cenomanian. Goretophylloceras Collignon, 1949 (type Ammonites fortunei Honnorat-Bastide, 1892), comprises thicker forms, with a less incised sutural line, with diphyllous or triphyllous saddles ending in large spatulate phyllites, at the base of which small internal phyllites are practically absent. It is limited to the Aptian and Albian stages.
Remarks. The genus Phylloceras is cosmopolitan, and its species existed from the Jurassic (Hettangian) to the Cretaceous (Maastrichtian). In Wright's Treatise (1996), Hypophylloceras is given genus status and includes the subgenera Aphroditiceras, Goretophylloceras, Hyporbulites, and Neophylloceras. Joly (1993, p. 10) points out that this definition is too broad, as it groups together forms that are very distantly related, primarily by their sutures. We follow his position here, which considers Hypophylloceras and Goretophylloceras to be two subgenera of genus Phylloceras. Interested readers can consult Joly (1998) to identify the Phylloceratidae of the Paris Basin and Joly & Delamette (2008) for the many species of southeastern France.