北海道の上部白亜系中部における二枚貝類 Mytiloides の層序的産出

抄録

Based on the stratigraphic investigations of the selected areas of Hokkaido (Fig.1), the successive occurrences of the following ten species of Mytiloides in the mid-Upper Cretaceous sequence have been clarified. The geological ages are determined by the associated ammonites and other well-known species of Inoceramus. They are here abbreviated as UC=Upper Cenomanian, LT, MT and UT=Lower, Middle and Upper Turonian, although the substages are defined tentatively. As to the definition of a species, we depend on current views of authors. In case of an unsettled taxon, we define the species on a number of specimens, admitting a considerably wide range of variation. (1) M. mikasaensis Matsumoto & Noda : UC Zone of Euomphaloceras septemseriatum; (2) M. cf. sackensis (Keller) : low LT; (3) M. columbianus (Heinz) : LT Zone of Pseudaspidoceras flexuosum; (4) M. goppelnensis (Badillet & Sornay) : LT (between 3 and 5); (5) M. mytiloides (Mantell) : mid-LT zone with Mammites sp.; (6) M. labiatus (Schlotheim) : upper LT (maybe lower or higher than 7); (7) M. subhercynicus (Seitz) : high LT; (8) M. hercynicus (Petrascheck) : isolated (highest LT or low MT); (9) M. teraokai (Matsumoto & Noda) : upper MT, Zone of Inoceramus hobetsensis, so far in Southwest Japan; (10) M. incertus (Jimbo) : UT Zone of Subprionocyclus neptuni-I. teshioensis. Some of the above species, e.g. (2), (6) and (8), are rare in the available material and need further hunting. (1) and (9) are so far endemic and should be globally searched for. However, the biostratigraphic succession of the Mytiloides species in our province is generally well correlated with that in the United States. Evolutionary and ecological problems are left for further studies.