84 
PACIFIC SCIENCE, Vol. XVII, January 1963 
species might be estimated. If in the future 
these areas are more thoroughly surveyed by 
quantitative methods, the faunistic differences 
or peculiarities will be more clearly demon- 
strated. For the same reason, present data are 
presumed to be insufficient for the following 
discussion and a supplemental report is to be 
expected. 
Geographical Differences of the Assemblages 
Within the Izu Banks, local differences in the 
assemblage are scarcely observed. The typical 
species-groups found in the area are roughly 
subtropical or warm-temperate forms from the 
lower part of the continental shelf bordering 
the main Japanese islands. On the other hand, 
probably because of the differences in latitude, 
a slight difference exists between the faunas of 
the northern half and those of the southern 
half of the Izu Banks. 
Geographically, the Izu Islands and the Osumi 
Group are distant from each other. Similar 
patches of banks are not found between the Izu 
Banks and the Osumi Banks. Therefore, the 
continuity or discontinuity of the fauna has 
not yet been studied. However, many species 
are found in both areas: Area mania takii 
Hatai, Niino, and Kotaka; A car congenitum 
(Smith); Samacar pacifica (Nomura and 
Zimbo); Haavaiarca uwaensis (Yokoyama); 
Mimarcaria aizoi Sakurai ( MS ) ; Striarca fausta 
Fig. 14. Chlamys mollita (Reeve) from Hyotanse, 
118 m, 18.5 mm X 17.0 mm. 
Habe; Chlamys vesiculosus ( Dkr. ) ; C. tissotii 
( Bernardi ) ; C. lemniscata ( Rve. ) ; C. mollita 
(Rve.); Spondylus anacanthus (Mawe); Pli- 
catula muricata (Sowerby); Frigido cardium eos 
(Kuroda); Chama argentata Kuroda and Habe; 
Tenagodns anguinus (L.) ; Bursa ranelloides 
(Rve.), var.; and Conus sp. 
Of the species that are not common to the 
two areas, some forms are apparently ecological 
equivalents of banks-associated forms. Several 
examples are shown here: 
IZU ISLANDS AREA OSUMI STRAITS AREA 
Perotrochus beyrichii (Hilgendorf) P. salmiana (Rolle) 
Galeoastraea guttata ( A. Ad. ) G, millegranosa Habe or 
G. tayloriana (Smith) 
Tucetona shinkurosensis Hatai, Niino, and Kotaka 
Lima fujitai Oyama L. tomlini Prashad? 
Gians sagamiensis Kuroda and Habe G. kyushensis, sp. nov. 
Possible Vertical Limits of the Banks-Associated 
Fauna 
The distribution of the benthic mollusca is 
governed much more forcefully by the sub- 
stratum of the depths than by other environ- 
mental factors. At equal depths in similar lati- 
tudinal position, or under the same ocean 
climate, the differences in representation are 
due to such bottom factors as mud, sand, gravel, 
and rock. The banks-associated molluscan as- 
semblages shows mainly hard-bottom facies 
mingling with some sandy bottom dwellers of 
the depths. This is closely associated with the 
insular shelf fauna, since the surrounding areas 
of almost all the small islands in the region are 
