Mollusca of Lake Biwa. 71 
donta is very abundant, but I did not bring away specimens and am not sure that it 
was not artificially introduced to serve as food for fishes at the fishery station. 
SUMMARY AND CONCLUSIONS, 
SUMMARY. 
Twenty-nine species of true aquatic Mollusca are known to live in Lake Biwa, while four others are 
common in ditches and small pools in the immecliate vicinity. The lacustrine species include twelve 
Gastropoda and seventeen Lamellibranchiata ; the non-lacustrine species, three of the former group and 
at least one of the latter. 
Of the lacustrine species, eleven (about 38%) are known only from the lake, and eleven others only 
from Japan; while the remainder (about 24%) occur also on the mainland of Asia. 
Eighteen (about 12%) of the lacustrine species are found on a muddy or sandy bottom in water less 
than 100 feet deep, but at least three of these species also stray into deeper water. 
Seven species (about 24%) are only found on stones or rocks near the margin. 
Seven species occur on a muddy bottom in water between 100 and 320 feet deep, but only three of 
these (less than 11% of the lacustrine fauna) are true deep-water forms abundant in depths over 260 feet. 
CONCLUSIONS. 
Lake Biwa, as might be expected from its geographical position and from what is 
known of the fauna of Japan generally, seems to be, so far as the Mollusca are 
concerned, the meeting place of two lines of migration, one from the north, the 
other from the south. The genera Choanomph^lus, Valvata and Pisidium are essen- 
tially northern genera, while such species as Nodularia hilirata and Corhicula sandai 
are southern forms, not known north of the lake. The rupicolous species, including 
those that adhere to stones, seem mostly to be endemic, but the peculiar stone-loving 
forms are small, inconspicuous and of furtive habits and may, therefore, have escaped 
notice elsewhere. 
Three of the other eighteen shallow-water species are also endemic, and two of 
the three true deep-water species ; but there are no endemic genera — unless the rupi- 
colous Lithotis japonica .should ultimately be proved generically distinct from the 
Indian species. 
The true deep-water forms belong to the genera Valvata and Pisidium and are of 
northern origin. They agree with European deep-water species of the same genera in 
their small size, pale colouration and fragility of shell, but the Pisidium. differs from 
its deep-water allies of both Europe and Siberia in having the umbones small and by 
no means prominent. 
The few shells of Corhicula taken in water over 250 feet deep were small, but 
short, inflated and relatively thick, though a species of the genus (C. tenuistriata , 
Prime) that has most of the peculiarities of the deep-water forms of Pisidium is found 
in fairly shallow but very muddy water in the Whangpoo River near Shanghai. 
No very exact comparison is yet possible between the deep-water Mollusca of 
Japanese lakes and those of the lakes of Europe and of the mainland of Asia, but, in 
Lake Biwa at any rate, deep-water forms of the two genera {Valvata and Pisidium) 
