Molliisca of Lake Biwa. 
6i 
reddish bands as in the type of M. retifera, Tryon. They were smaller than speci- 
mens from the hills on the western side of the lake and from ditches at Hikone. The 
shells from Hikone were much darker. The following are the measurements of large 
examples from the lake at Zeze, from a ditch at Sakamoto and from a ditch at 
Hikone respectively. 
Melania wbertina. 
Length. Maximum breadth. Condition. 
Zeze (Lake Biwa) . . . . 19 mm. 8 mm. Tip eroded. 
Hikone (ditch) . . . . 20 mm. 12 mm. Tip slightly 
eroded. 
Sakamoto (ditch) .. .. 30 mm. 12-5 mm. Tip eroded. 
It is probable, therefore, that M. lihertina is not, strictly speaking, a lacustrine 
species, for it does not attain its full development in the lake. 
The typical form of Bithynia striatula appears to be, at any rate in central China, 
a true lacustrine form ; it is common in the Tai-Hu of the Kiangsu Province on 
stones near the margin. I am doubtful, however, whether the Japanese race has the 
same habits. It has been found at places where there are no large lakes, and my 
own specimens from Lake Biwa were taken in a kind of backwater. It is very prob- 
able, therefore, that it should be included among the non-lacustrine forms found 
close to Lake Biwa and occasionally straying into the lake. 
There is no evidence that the restricted bathymetric range of the rupicolous 
species of Lake Biwa is due in any way to depth of water, for the peculiar environ- 
ment that suits them happens to occur there at the margin or around islands, and 
any rocks that may have existed in the deeper depressions have long ago been buried 
in mud. 
Except where the shore is stony or rocky there are extremely few molluscs to be 
found in the water less than about 10 feet deep. This may be due, partly at any 
rate, to the careful habits of Japanese fishermen, who do not permit even small 
animals that they consider edible to escape their notice. 
SHALLOW WATER FORMS. 
The nature of the bottom of Lake Biwa, even in comparatively shallow water, 
differs considerably, as I have already indicated, in different parts of the lake. The 
southern region is for the most part muddy and produces so dense a growth of weeds 
that they are raked into large boats and spread out as manure on the fields. To- 
wards Seta in the south-east corner, however, considerable quantities of sand are 
mixed with the mud and the weeds are much less luxuriant. The eastern side of the 
lake is muddy, while beaches of very coarse sand extend along a great part of the 
western shore and outwards into water over 100 feet deep. 
Less variation in the Molluscan fauna is correlated with this variation in environ- 
ment than might be expected. Limnaea pervia, a scarce species, has only been found 
among weeds and Corbicula sandai on a bottom of muddy sand, on which C. viola is 
