Preliminary Notes on Molluscan Assemblages of the Submarine Banks 
Around the Izu Islands 
Takashi Okutani 1 
It is well known that there are several sub- 
marine banks along the submerged rise which 
extends southwestward from the southern tip 
of Izu Peninsula, central Honshu. Small islands 
such as Toshima, Niijima, Shikine, and Kozu, 
with several other islets, lie on this rise. These, 
together with a few other islands situated far- 
ther south, are called the Izu Islands. They are 
linked by a volcanic system, and there is a con- 
siderable number of such banks in the neighbor- 
hood. A few papers concerned with hydrograph- 
ical, bathymetrical, and faunistic characteristics 
of these submarine banks have been prepared by 
Suzuki and Sato (1944), Niino (1935, 1952, 
1955), and Shirai (1958). On the basis of these 
works, together with information furnished by 
the present author, Horikoshi (1957) discussed 
the topographical peculiarity in relation to the 
general molluscan fauna on these banks. 
Another group of submarine banks is found 
around the O'sumi Islands, south of Kyushu. 
Presumably their hydrographical and bathymet- 
rical characters are similar to those banks men- 
tioned above, but no information about the 
molluscan fauna has been available until now. 
As a contribution to knowledge about mol- 
luscan fauna on the submarine banks and in- 
sular shelves around the Izu Islands, this paper 
deals with the general account of the molluscan 
assemblages of the area and their faunal simi- 
larity to another series of banks near the Osumi 
Islands in the Kuroshio area. It is based on 
biological dredge samples collected chiefly by 
research vessels during 1955-59. 
The present writer wishes to express his grati- 
tude to Dr. Z. Nakai, Tokai Regional Fisheries 
Research Laboratory, for the material. Thanks 
are extended to Mr. Y. Kurata, Tokyo Fisheries 
1 Tokai Regional Fisheries Research Laboratory, 
Tokyo, Japan. Manuscript received September 11, 
1961. 
Experimental Station; Dr. M. Horikoshi, Ocha- 
nomizu University; Dr. K. Sakurai and Mr. A. 
Teramachi, members of the Japan Malacologi- 
cal Society, for their facilities and advice ren- 
dered in the course of this study. Thanks are also 
due the crews of the research vessels for their 
cooperation in collecting the present materiai. 
TOPOGRAPHY OF SUBMARINE BANKS AROUND 
IZU ISLANDS 
The Izu Isiands extend from the mouth of 
Sagami Bay to the south. In the northern group 
are (Izu-)Oshima, Toshima, Niijima, Shikine, 
and Kozu. To the south there are Miyake, Mi- 
kura, Hachijo, Aogashima, and Torishima as 
the southern extremity (Figs. 1, 2). The banks 
are usually isolated from the series of these is- 
lands, with depressions deeper than 200 m lying 
between them. The tops of such types of banks 
are usually flat and about 80-120 m in depth. 
They are usually elongate-oval in shape with 
the axis in a northeast-southwest direction. 
The Hyotanse Bank, one of the representa- 
tives of this series of banks, located west of 
Kozu Island, has been described by Niino 
(1955) as follows: The slopes around the bank 
are steep and rocks are exposed there; gravels 
and coarse material cover the broad and flat 
plain on its top; andesite and basalt, which are 
very common in the bedrock, are found mingled 
with liparite gravels together with a number of 
manganese concretions from the bank; the litho- 
logical characters of these rocks are the same 
as those of the main islands in the Fuji Vol- 
canic Zone. According to gross observation of 
the present material, the sediments (gravels 
and shells) are heavily coated by calcareous 
algae. 
The bottom of Zenisu Bank, also studied by 
Niino (1935), reveals coarse sand and shells, 
73 
