Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum No. 3

Polyplacophora fossils collected from the Pleistocene formations of the Boso and Miura Peninsulas, environs of Tokyo, are studied.

Thirty species of eight genera in seven families are recognized in about 2000 specimens from eight localities as Tsujimoribashi (A), Tsujimori (A'), Koshiba (B), Nako (C), Dai

(D), Tsukui (E), Semata-no-seki (F) and Sakurai (G) as shown in Table 1.

Polyplacophora assemblages in these formations are mixed one consisting of warm sea type species as Onithochiton hirasei and Afossochiton sp. etc., and cold sea type species as Mopalia schrenckii and Placiphorella stimpsoni etc. There are differences in constituents in each beds as warm sea type species are predominant in Sakurai and Sakahata beds. Geochronological change in character of assemblage is recognized as follows;

S-(N) (earliest) → N-S (early ) → S-N (middle) → S-(N) (late)

(S: warm sea type ; N: cold sea type)

Occurrence of the species as Afossochiton sp. of which correlatives are found in Miocene　to Pliocene series of Victoria, Australia and "Notoplax" sp., a related species with Notoplax foresti living in the Gulf of Guinea, Africa, is a remarkable fact.

Above-cited facts on Polyplacophora fossils bring foward a zoogeographical problem in viewpoint of its origin and migration in age from Miocene to present in Japan.

Each species is described in Japanese. Some problems on taxonony are discussed and concluded as follows;

1. Genus Afossochiton belongs to Family Acanthochitonidae in conformity with opinions of Ashby and Cotton (1939), and Cotton (1964).

2. "Notoplax" sp. has primitive valve structure and belongs to a species group of Notoplax which is different from the group of Notoplax species living in Japanese seas. It has possibility to belong to another new genus or subgenus.