Molluscan Assemblages — Okutani 
85 
almost entirely rocky. On the other hand, this 
is not always similar to the shelf fauna of the 
main islands because the shelf bordering the 
main islands does not always present a hard 
bottom. For instance, a molluscan assemblage 
from the soft bottom within Sagami Bay at a 
similar depth ("Soyo” sta. T2 6, Nov. 15, 1958, 
35° 15.4' N, 139° 27.9' E, 102 m) is made 
up of: Glycymeris rotunda (Dkr.) ; Delecto- 
pecten macro chrili cola Habe (believed to be 
found attached to the carapace of the giant 
spider crab, but recently recovered as free living 
individuals ) ; Venus faveolatus Sowerby; Onus- 
tus exutus (Reeve); Granulifusus niponicus 
(Smith). Moreover, collections from a certain 
area in Sagami Bay sometimes contain species 
similar to those from the banks under study 
(cf 12 and 13), while a different assemblage 
is found from another hard bottom in the Bay 
at the same level. Therefore, the most remark- 
able character of a banks-associated assemblage 
is the constant occurrence and constant domi- 
nancy of the typical species of the area. 
The typical banks-associated assemblage ap- 
pears at depths ranging from 32 m as the shal- 
lowest to 290 m as the deepest, centering around 
100-250 m. In waters shallower than this, a 
certain upper-shelf assemblage is observed, 
which is frequently collected by a lobster-net or 
in Gelinidium collecting. Examples of hard- 
bottom dwellers in the shallower zones of the 
Oshima Islands are: Turbo cornutus (Solander); 
F usinus nicobaricus (Lamarck); Fasciolaria gla- 
bra (Dkr.); F. trapezium audouini (Jonus), 
Fig. 15. Tucetona shinkurosensis Hatai, Niino, and 
Kotaka from Niijima-Nishitai, 14 mm in height. 
Fig. 1 6 . Tucetona hanzawai (Nomura and Zimbo) 
from Kikaijima (fossil). 
etc. The sandy bottom facies of the upper shelf 
in Okada, Oshima Island, is reported by the 
dominance of the following species: Terebra 
nebulosa Sowerby; Conus tes sulatus Born; 
Chlamys vesiculosus (Dkr.); Callista pilsbryi 
Habe; Dosinia iwakawai Oyama and Habe; An- 
tigona lamellaris Schumacher; Sunetta concinna 
(Dkr.), var.; Paphia amabilis (Philippi). 
Judging from these observations, the ecotone 
between a shallow insular shelf fauna and a 
lower insular shelf fauna, which is almost equi- 
valent to banks-associated fauna, is presumably 
at a depth of 50 m or so. The deeper limit may 
be about 300 m. For example, at a station near 
Okinoyama, about 300 m deep, the following 
species were collected from a gravel bottom 
("Soyo” sta. T20’C, Aug. 11, 1958, 34° 59.0' N, 
139° 32.3' E) despite the fact that the similar 
banks-associated assemblage is observed on the 
upper part of this bank: Chlamys sp. aff. mollita 
(Rve.); Delectopecten macrocheilicola Habe; 
Keenaea sakuraii Habe; Pandora sp.; Turcicula 
crumpii Pilsbry; Trophonopsis echinus (Dali); 
Japonacteon archibenthicola Habe. Gravels from 
such depths are not covered by calcareous algae. 
This archibenthal assemblage almost agrees 
with that found from similar depths near Omu- 
rodashi reported by Igarashi and Kurata (1956). 
As has been stated, the molluscan community 
on the islands near the banks under study is 
allied to those from neighboring banks; while, 
out of biological dredge samples from stations 
lower than that, at a depth of 470-480 m, near 
Hachijo Island, a different archibenthal mollus- 
can assemblage is found ("Soyo” sta. B3, March 
