Abstract
A peculiar ammonite recently acquired by Masatoshi Kera from the Coniacian rock of the Masago-zawa in the Yubari Mountains, central Hokkaido, is described as a new species. It may represent the earliest species of Eupachydiscus Spath, 1922, retaining some features which characterize certain species of Tongoboryceras Housa, 1967. In this connexion Tongoboryceras is redefined so that it may include comprehensively more species. Also Menabonites Housa, 1967 is reviewed with a proposal to include it in Pseudojacobites Spath, 1922 in a revised sense. In the available material there is no significant difference of size, i. e. a microconch versus macroconch relationship, between bior trituberculate species of Pseudojacobites and 'normal' pachydiscid species of Tongoboryceras, although the two genera are closely allied to each other.
