86 
PACIFIC SCIENCE, Vol. XVII, January 1963 
20, 1958, 33° 07.5' N, 140° 03.2' E, 480 m and 
do. Nov. 17, 1958, 33° 10.0' N, 140° 02.7' E, 
470 m): Fusinus? sp.; Trophonopsis echinus 
(Dali); Nep tunea constricta (Dali), var.; Ben- 
thovoluta sp.; Conus sp. nov. The bottom is 
found to be of volcanic gravel which is not 
coated by algal matter. 
SUMMARY AND CONCLUSION 
As the result of examination of biological 
dredge samples taken from submarine banks 
around the Izu Islands, it became clear that pos- 
sible characteristic banks-associated molluscan 
assemblages are found in this area. Since they 
are closely associated with the bottom nature 
and the depth of the banks, the assemblages are 
composed mostly of the hard-bottom lower shelf 
fauna found on the southwestern Pacific coast 
of the Japanese main islands. 
The present material was collected at random 
with various kinds of gear, so that a quantita- 
tive analysis of the fauna was not possible. How- 
ever, constant occurrence and relative abundance 
of certain species were assumed as indicators 
of the fauna of the areas. The typical banks- 
associated molluscan assemblages from the Izu 
Islands area are possibly represented by such 
gastropods as Perotrochus beyrichii, Galeoastraea 
guttata, and Tenagodus anguinus, together with 
several pelecypods such as A car congenitum, 
Area mauia takii, Hawaiarca uwaensis, Samacar 
pacific a, Mimarcaria aizoi, Tucetona shinku- 
rosensis, Chlamys vesiculosus, C. tissotii, C. lem- 
niscata, C. mollita, Spondylus anacanthus, Plica- 
tula muricata, and Lima fujitai. 
Fig. 17. Chama argentata Kuroda and Habe from 
insular shelf of Takeshima Is., 11.6 mm X 10-1 mm. 
Fig. 18. Spondylus anacanthus (Mawe) from Uji 
Islet, 126 m, 40.2 mm X 37.5 mm. 
The geographical difference of the assemblage 
within the area is not so obvious, but it is clear 
that the insular shelf fauna is substantially the 
same as that under discussion. It seems that the 
species found in the area are those distributed 
also in warmer districts. This impression may 
be corroborated by facts such as these: 
1 ) Similar banks-associated assemblages are 
found in southern waters off Kyushu 
which are much more subtropic in average ' 
climate. 
2) Tucetona hanzawai and Samacar pacifica, 
which are found from the Izu Islands area, 
were originally described by Nomura and 
Zimbo (1934) from the Ryukyu Lime- j 
stone of Kikaijima (or Kikai-ga-shima) , 
an island located far to the southwest of ■ 
the Osumi Group. Concerning the fossil 
fauna found from this Ryukyu Limestone, j 
Nomura and Zimbo cited Yabe and Han- 
zawa’s opinion that the Ryukyu Lime- j 
stone was deposited in waters warmer 
than are found there at the present time, 
and that fossils found there are very sim- 
ilar to the recent fauna found around the j 
