Douvilleiceratid ammonites from the lower to middle Albian (Lower Cretaceous) Yezo Group of Hokkaido, Japan, and a revision of the genus Douvilleiceras

The early to middle Albian genus Douvilleiceras from the Yezo Group of Hokkaido, Japan, is examined systematically. Five species referred previously to the genus are identified as D. mammillatum (Schlotheim, 1813), D. inaequinodum (Quenstedt, 1846), D. cf. offarcinatum (White, 1887), D. alternans Casey, 1962, and D. sp. A revision is made of the genus Douvilleiceras, and 40 species described in the literature are reduced to 7 on morphological criteria, including mode of ribbing, shape of the ventral sulcus, and mode of tuberculation, particularly a distinct difference in the number of tubercles on the primary ribs of the middle growth stage. The genera Trinitoceras and Eodouvilleiceras, which have been regarded previously as junior synonyms of Douvilleiceras, are morphologically distinguishable from the genus Douvilleiceras and hence considered valid.