Mollusca of lyake Biwa. 
63 
extends in I^ake Biwa from between 2 and 3 to 41 fathoms. As will be shown later, 
shells from the greatest depth are dwarfed and the genus is not common at most 
places below 100 feet. I obtained no evidence that Anodonta, Lanceolaria, and 
Nodularia went deeper than about 6 fathoms. 
These results, imperfect as they are, may be compared with those obtained by 
the Japanese fishery experts, with which, in most respects, they are in general agree- 
ment. In the map published in the pamphlet to which I have alluded at the begin- 
ning of this paper, Corbicula is not shown as occurring at depths much greater than 
30 fathoms. But the map was prepared largely from a commercial point of view, and 
from the fact that the authors were apparently unacquainted with the existence of 
Pisidium and Valvata in the lake it would seem that they did not explore the greater 
depths. They show Cristaria as descending to about 30 fathoms, while Nodularia 
and Lanceolaria, which they did not distinguish one from the other, are represented 
as most abundant between 6 and 10 fathoms. Hyriopsis is recorded by them from 
comparatively few localities on the east side of the lake, between 6 and 10 fathoms 
and in quite shallow water. There is no evidence available as to the bathymetric 
range of Pseudodon in the lake. The single species that occurs (P. loomisi) is prob- 
ably scarce. 
Taking the Japanese records, which are based on long-continued local investiga- 
tions, into consideration with my own results, the following facts become evident as 
to the shallow-water species, i.e., those found in depths less than 100 feet. 
The majority of the Mollusca found in Lake Biwa occur between 2 and 17 fathoms. 
From this zone no less than 16 species and 9 genera have been definitely recorded. 
The majority of the species (11) belong to the family Unionidae, of which no less 
than 6 genera are included. With the exception of Cristaria plicata, all the species 
of this family are apparently true shallow- water forms not occurring below 100 
feet. 
Although the Unionidae thus predominate so far as number of species is c:on- 
cerned, it is probable that the majority of individual molluscs present in the lake 
belong to the three species Melania multigranosa , Vivipara 
Predominant species. • i ^ 7 • 7 • 7 tw-t <• 1 • • 1 
sclateri and Corbicula viola. None of these species are strictly 
confined to water shallower than loo feet. Their precise bathymetric range will, 
therefore, be considered with that of the deep-water forms. Corbicula sandai has been 
taken only in shallow water, but it is apparently very scarce in Lake Biwa. The 
precise distribution of the different species of Unionidae must be left for local obser- 
vers to elucidate. 
DEEP-WATER FORMS. 
In some respects the deep-water fauna of Lake Biwa is more interesting than that 
of any other zone, for few particulars are available in reference to deep-water forms 
from other lakes in Asia. I may say now, that when the complete results of my in- 
vestigations in the Japanese lake are published they will prove that the Mollusca are 
by no means the only groups in which distinct deep-water forms occur, for this is also 
