196 EIGHTH PACIFIC SCIENCE CONGRESS 



b) Molluscs which appear in a few bays at present but are sup- 

 posed to have had a wider distribution range in Japan, though no fossil 

 remains are at hand. They are common or represented by closely re- 

 lated species in the continental coast. 



Glaucome chinensis: Tokyo Bay, Kozima Bay of the Inland Sea,. 



and Ariake Bay. 

 Sinonovacula constricta: Kozima Bay and Ariake Bay. 

 Striarca olivacea: Tokyo Bay, Kozima Bay, and Ariake Bay. 

 Siandella pellucida: Kozima Bay. 

 Cerithidea largillierti: Ariake Bay and Kozima Bay. 

 Nassarius varicijerus: Inland Sea and Northern Kyushu. 



c) Molluscs which appear as fossils in Japan, but are represented 

 by closely allied species in the continental coast. 



Protapes irrediviva: Totuka not far from Yokohama. 



Mabellarca hiratai: an undescribed new Ark-shell from Kagoshima of 

 southern Kyushu. 



These facts seem to prove that the continental coastal elements of 

 bay fauna have recently declined or become extinct. The causes of 

 decline are not clear. The possible difference of submarine climate at 

 different localities is not large enough to explain this phenomenon. 

 As a more plausible reason for the decline of this fauna, which belongs 

 to muddy bottom community especially of indentation of bays, may be 

 mentioned the recent change of micro-topography of the Japanese coast. 



N. Sakazume (1952) has found that Anadara granosa, a bivalve 

 now extinct in Tokyo Bay, is an important member of the shell mounds 

 along the coast of prehistoric Tokyo Bay. It occurs more abundantly 

 in older mounds situated near the head of indentations. As the landing 

 of the bay proceed, the sites of mounds advance toward the mouth of 

 indentations, where this shell becomes a less prominent member of the 

 shell assemblage. It has also been confirmed that in the same site, the 

 lower strata of a mound contain more A. granosa than the upper strata. 



These facts seem to prove the gradual decline of this bivalve with 

 the progress of landing process in recent geological time. In our opin- 

 ion, the declining molluscs are preserved as relics in such places as 

 Ariake Bay and a part of the Inland Sea (Kozima Bay as a typical place), 

 where muddy bottom has still a large extent. In Ariake Bay, for an 

 example, the tidal area is so muddy that shell collection by fishermen 

 is made by using wooden runners or mud ski. The declining process 

 seems in progression at present. 



Thus in this case the supposed existence of a special micro-province 

 is to be explained neither by the geographical proximity nor by the 

 climatic similarity, but by the survival of some continental elements 

 in a special habitat. 



