MARINE BIOGEOGRAPHICAL MICRO-PROVINCES — JAPAN 495 



related forms in the continent. Corbicula japonica and Assiminea jap- 

 onica may be examples of this category. 



From these facts it may be concluded that different parts of the 

 same littoral region with similar depth may belong to different bio- 

 geographical provinces according to the kind or system of fauna on 

 which they are based. 



Recent Decline of Continental Coastal Fauna from Japan 

 and its blogeographical meaning 



Among factors to determine the biogeographical provinces, there 

 are geographical distance and ecological conditions of habitat on the 

 one hand and historical processes on the other. The analytical study 

 of the relative weight of these factors is, however, not always easy. 



In Ariake Bay on the west coast of Kyushu, there occur several ani- 

 mals which do not appear in other districts of Japan but are very com- 

 mon in the continental coast of Asia. An Enraulid Coilia ectensis and 

 a Gobioid Boleophthalmus pectinirostris among fishes and Salinator 

 takii (represented by allied forms in the continent), Assiminea latericea 

 and Cerithidea ornata among snails are some of the examples of this 

 category. Based on these facts, Ariake Bay has been regarded to form 

 a special biogeographical micro-province, and this peculiarity was attri- 

 buted to the geographical proximity between this bay and the continent. 

 Our recent survey has revealed that there are quite many mollus- 

 can species which are found in only a few bays often including Ariake 

 Bay, but are very common in the continental coast. Some of these 

 molluscs are found as semi-fossils in many shell mounds and recent 

 sediments along the Japanese coast. Some of the examples are as fol- 

 lows: 



a) Molluscs found in a few Japanese bays at present, but their wide 

 pre-historic distribution has been proved by remains in shell mounds 

 and deposits: 



Anadara (Tegillarca) granosa: Living in Ariake Bay, Inland Sea 

 and Mikawa Bay. Widely distributed in the continental coast. 

 Fossil or semi-fossils in shell mounds or deposits of Sendai in 

 North Japan, Nanao of Japan Sea coast and Osaka Bay. 

 Trisidos tortuosa kiyonoi: Living in northern Kyushu (Fukuoka 

 Bay and Karatu Bay). Represented by T. tortuosa in Indo- 

 China and China coast. Fossils from Toyohasi in Central 

 Japan and Osaka Bay. 

 Erodona amurensis: Distributed in Hokkaido, Korea, China and 

 Amur Region. Fossils from Toyohasi in Central Japan and 

 Osaka Bay. 



