Molluscan Assemblages— OYJJTANl 
83 
there are 25 rounded ribs in the latter species; 
the interspaces of shinkurosensis ribs are much 
narrower than the ribs, while the interspaces of 
the latter species are nearly equal to the ribs; 
the hinge teeth of the former are 5 in the an- 
terior half and 6 in the posterior half, instead 
of 8 and 9 on the anterior and posterior half 
of the hinge plate of the latter species. 
In the present survey, 40 odd valves from 
Izu Banks were examined. The size ranges of 
the material were 7.7-17.4 mm in length and 
7.6-18.1 mm in height. Of these specimens, no 
valve was found which had rib interspaces as 
wide as the ribs themselves, despite the fact 
that meristic characters vary with individuals, 
i. e., height/length, ranges 0.915-1.090; num- 
ber of radial ribs, 24-33; anterior hinge teeth, 
5-10; posterior teeth, 7-11. On these evi- 
dences, all of the specimens are identified as 
T. shinkurosensis. 
The measurements of two odd valves of topo- 
types of T. hanzawai are: 
HEIGHT 
HEIGHT/ LENGTH 
RADIAL RIBS 
TEETH 
Ant. Post. 
No. 1 (left valve) 
11.1 mm 
1.057 
29 
7 
7 
No. 2 (right valve) 
12.0 mm 
1.190 
25 
8 
7 
The interspaces of radial ribs are estimated 
to be about half as wide as the ribs. In compari- 
son with specimens of two species of similar 
size, T. hanzawai is provided with more distin- 
guishing features than are pointed out in the 
foregoing lines: it has a more prominent umbo, 
a deeper shell, and less angular shoulders, thus 
a V asicardium- like appearance, partly because 
of its shell which is longer than it is high. Nev- 
ertheless, the morphological similarity of these 
two requires further biometrical study in the 
future. 
18. Chama argentata Kuroda and Habe 
Fig. 17 
This is found on many banks and insular 
shelves under study. It was collected from the 
Niijima-Nishitai Banks, the insular shelf of 
Torishima, the Uji Islets, insular shelves of 
Takeshima (Sta. 114), and the Goto Islands 
(Fig. 20c). 
Other species than those enumerated above, 
Spondylus anacanthus (Mawe), Plicatula muri- 
cata ( Sowerby ) , and Lima fujitai Oyama ( Figs. 
18, 19, and 20d), are usually found abundantly 
in the area. The occurrence of two or three of 
them may be a remarkable faunal characteristic 
of the banks. 
GEOGRAPHICAL DIFFERENCES AND VERTICAL 
LIMITS OF TYPICAL BANKS-ASSOCIATED 
ASSEMBLAGES 
Since the present material was obtained with 
different kinds of gear from a limited number 
of stations, a conclusive quantitative analysis 
can not be made at present. However, the abun- 
dance or probable dominancy of the several 
Fig. 13. Chlamys lemniscata (Reeve) from Uji 
Islet, 126 m, 22.0 mm X 20.0 mm. 
