Mollusca of lyake Biwa. 57 
Pisidmm casertanum is a ver}^ widely distributed Palaearctic species, extremely 
variable and therefore burdened with a vast synonomy. On the mainland of Asia it 
has only been found as yet in I^ake Baikal. It may be regarded as distinctly northern 
in origin. 
Thus, if we consider these seven mainland species together with those that are 
endemic either in the basin of Lake Biwa or in Japan as a whole, we find strong evi- 
dence of the two lines of migration to which I have already referred more than once. 
Lanceolaria bilirata, Corhicula sandai and possibly Lithotis japonica and Bithynia 
striatula are southern forms, while Pisidmm casertanum, the two species of Valvata — 
it should be noted that these three are deep-water molluscs in Lake Biwa— and pos- 
sibly Hyriopsis schlegeli and Choanomphalus represent an immigration from the north 
Part in. BIOLOGICAL DISTRIBUTION. 
In this section of my paper I propose to consider the distribution of the mol- 
luscan fauna of Lake Biwa into various life-zones. Before doing so, however, it will be 
necessary to give a few particulars as to the physical characters of the lake (see map 
on next page). For these particulars I am indebted largely to a little guide-book in 
EngHsh published by the authorities of the Shiga Prefecture. The geographical 
statements in the book are, I believe, founded largely on the investigations of the 
officers of the Prefectural meteorological and fishery stations at Hikone, to whom 
I was also indebted for valuable verbal information. Much of this information is 
otherwise available only in Japanese. 
Lake Biwa has a superficial area of 269 square miles and a circumference of 144 
miles. It is divided into two regions, — a southern region in which the water is shallow 
and choked with weeds, and a much larger northern region, 
Physical characters of Lake ,1 . -ri-*ii- jj.ir r.i.T„ 
the greater part oi which has a depth or over 200 reet. In 
Biwa. 01. J. ^ 
this latter region there are two deep depressions of considerable 
area in which depths of over 250 feet are recorded ; at the deepest point the depth is 
320 Japanese feet ( =97 metres). In the map on p. 58 the depths are shown in Japanese 
feet, according to the investigations of the Shiga meteorological and fishery bureaus. 
The bottom is for the most part muddy, especially near the centre of the lake, 
but at certain places there are beaches of coarse sand^ which extend out into the 
water for a considerable distance from the shore. In the northern region there are 
one or two small rocky islands and at several places, notably near Otsu at the south 
end of the lake, the margin is covered with small stones and broken pottery, derived 
mainly from old buildings and the like. 
The 136th parallel of eastern longitude runs through the lake, while the 35th of 
northern latitude passes a few miles to the south. 
The water of Lake Biwa is remarkably clear and free from sedimet. The mean 
summer temperature near the shore is 33°C. and out in the middle 30°C. In the 
greatest depths the temperature falls as low as 8°C. at that season. In winter the 
average temperature is 8°C. It has been calculated that 33 % of the waters of the 
