It is well known that coefficient of closeness (C=100c/√S₁S₂), which is the ratio of the number of common species (c) between two different localities to the geometric mean of their respective total number of species (S₁, S₂) proposed by O_TSUKA, is useful in precisely comparing the faunistic feature between two different localities. With a view to our clarifying the distributional pattern of each species, degree of popularity (P) is newly defined herein in the following formula:
P=N/S(n-1)
Where, N stands for the frequency that one species is concurrently obtained together with other species of this group from the same locality in each zoogeographic division, n for the total number of species examined and S for the number of investigation.
The results obtained are summarized as follows:
(1) According to the coefficient of closeness among Nagasaki, Matsue, Maizuru, Niigata, Onomichi, Numazu, Kominato, Matsushima, Muroran, Owashi, Kochi and Kagoshima (Tab. 1), the Japanese waters can be divided into 5 faunal subdivisions. The southern part of Japan from Ryûkyû to Suruga Bay on the Pacific is geographically characterized by the fact that all Japanese soles including 7 tropical species never before found in any other part of Japan are unfailingly found there. The East China Sea is easily distinguished from other regions by the presence of characteristic species Zebrias fasciata.
On the Japanese coast of the Japan Sea and the Pacific coast of the middle part of Japan (from Sagami-Nada to Kashima-Nada) several temperate and subtemperate soles are known. The fauna of the soles in the Inland Sea of Japan is of peculiar in character, being more or less influenced by the neighbouring fauna. But the northern part of Japan on the Pacific is the most sterile region, so far as soles are concerned, only 3 species (Areliscus interruptus, Zebrias japonicus and Rhinoplagusia japonica) being reported there.
(2) The degree of popularity is large in Areliscus interruptus, Aseraggodes kobensis, Zebarias japonicus, Heteromycteris japonicus and Aesopia cornuta than in any other species found in the southern part of Japan on the Pacific (Tab. 2). On the Japanese coast of the Sea of Japan and the Pacific coast of middle part of Japan, the degree is large in Areliscus interruptus, Zebrias japonicus, Rhinoplagusia japonica and Areliscus joyneri (Tab. 3).
(3) On the whole, the temperature and the subtropical forms of this group, whose degree of popularity is large, are widely distributed even to the northern part of the Japanese waters. But the tropical form has the degree usually smaller than 0.1, the northern limit of its range lying in the southern part of Japan.