54 
ZOOLOGY OF THE FAR EAST. 
Part II GEOGRAPHICAL DISTRIBUTION. 
The majority of the molluscan genera found in Lake Biwa fall into one of three 
geographical categories — either they are practically cosmopolitan in distribution, or 
they, have a wide range in the warmer regions of the world, or else they are found on 
the mainland and islands of eastern Asia. 
To the first of these categ,ones belong Limnaea , Planorhis , Bithyni a , Vivipara, 
Sphaeriuni and Pisidium. The tropical and subtropical genera not restricted in range 
geographically include only M^/awm and Coy&icw/rt ; while Hyriopsis, Cristaria, Ple- 
tholophus, Lmceolaria, Nodularia and Pseudodon are characteristically eastern Asiatic. 
Anodonta and Valvata are essentially Palaearctic genera, though species of the 
former occur in tropical Asia. The range of Lithotis and Choanomphalns is evidently 
as yet but imperfectly Vnown. The former has been recorded only from Peninsular 
India and from Lake Biwa; while the latter, best known from Lake Baikal, occurs 
also in the eastern Mediterranean region and possibly on the Pacii&c side of North and 
South America. 
Of the cosmopolitan genera it is unnecessary to say more at present : of the two 
tropical and subtropical genera it may be noted that in both cases the range extends 
rather further north in eastern Asia than it does in the West. The eastern Asiatic 
genera seem to represent two elements, one certainly southern and the other possibh'' 
northern in origin. The double immigration will become clearer if we consider with 
these mainland types the Palaearctic genera Valvata and Anodonta, and also Pisidium, 
the range of which is somewhat anomalous so far as Japan is concerned. Anodonta, 
it is true, does not provide strong evidence, for some species are widely distributed 
over the warmer parts of the Far East. Cristaria, however, appears to be mainly 
northern ; Hyriopsis, Lanceolaria, Pseudodon and possibly Nodularia mainly southern. 
Valvata is only known in Japan at present from two deep lakes in the Main Island ; 
its species are found in Europe, Northern Asia and North America, and it is probable 
that no true representative occurs in the tropics. Pisidium, though several species 
are found in tropical Asia, is only known as yet, so far as the Japanese islands are 
concerned, from the almost sub- Arctic Hokkaido and from Lake Biwa. It is possible 
that the small size of the shell has caused it to escape notice in other localities, but 
the abundance of P. casertanum in the deeper parts of Lake Biwa, though it does not 
entirely preclude this point of view, is nevertheless opposed to it. 
The status of Lake Biwa as the meeting place of two lines of migration from 
the north and from the south respectively will become still clearer when we discuss the 
distribution of species as distinct from that of genera. 
It is convenient to consider the species under three headings, to include (i) 
those that are apparently endemic in the basin of the lake, (2) those that are endemic 
in the islands of Japan, and (3) those that are found also on the mainland of Asia. 
They may therefore be listed as Endemic," " Japanese" and Mainland." Omit- 
ting for the present varieties and local races (subspecies), 33 species of aquatic Mol- 
lusca are known to occur in the lake and its immediate surroundings. As will be seen 
