927. ON SOME ACANTHOCERATID AMMONITES FROM THE TURONIAN OF HOKKAIDO : STUDIES OF THE CRETACEOUS AMMONITES FROM HOKKAIDO-LXIX

Abstract

New or revised information is given on the taxonomy and stratigraphic occurrence of the following five species : (1) Pseudaspidoceras flexuosum Powell, 1963, which has recently been found at four localities in the lowest part of Member IIn, i.e. basal Turonian, in the Oyubari area. One of the examined specimens was the holotype of Pseudaspidoceras (Ampakabites) kawashitai Matsumoto and Obata, 1982, which now falls into synonymy with Ps. flexuosum. (2) Neomphaloceras pseudomphalum (Matsumoto, 1975), whose revised diagnosis is given. It occurs in the lower part of the Middle Turonian. (3) Neomphaloceras costatum (Matsumoto and Kawashita, 1978), which was erroneously assigned to Mammites or Morrowites. It occurs in the upper part of the Middle Turonian. (4) Mammites aff. nodosoides (Schluter), which is more evolute than typical M. nodosoides. (5) Mammites n. sp. (?), which has peculiar bar-like ribs across the venter and spatulate ventrolateral horns on the body chamber.