Molliisca of Lake Riwa. 
55 
from the three lists that follow, ii of these are known only from the basin of Lake 
Biwa, and 15 from Japan ; while 7 have been found also in eastern continental Asia. 
Endemic. 
Lithotis japonica. 
Choanomphalns japonicus. 
Planorbis (Gyraulus) biwa- 
ensis. 
Melania multigranosa. 
Melania niponica. 
Melania hiwae. 
Valvata hiwaensis. 
Valvata annandalei . 
Nodular ia hiwae. 
Nodularia reiniana. 
Corbicula viola. 
Japanese. 
Limnaea japonica. 
Melania libertina. 
Vivipara japonica. 
Vivipara malleata 
Vivipara sclateri. 
Hyriopsis schlegeli. 
Cristaria spatiosa. 
Pletholophus reiniana. 
A nodonta calipygos . 
Lanceolaria oxyrhyncha. 
Nodularia japanensis. 
Nodularia hirasei. 
Nodularia parcedentata. 
Pseudodon loomisi. 
Sphaenum heterodon. 
Mainland. 
Limnaea pervia. 
Bithynia striatula. 
Cristaria plicata. 
Anodonta woodiana. 
Lanceolaria bilirata. 
Corbicula sandai. 
Pisidium casertanum. 
Of the species apparently endemic in the lake two hold a pecuhar position on 
account of the genera to which they belong, viz. . Lithotis japonica and Choanompha- 
lus japonicus. The shells are, however, in both cases very small and inconspicuous 
and the animals live concealed on the lower surface of stones, a position in which 
they may very well have escaped notice in other localities. This is also true of 
Planorbis [Gyraulus) biwaensis. Valvata biwaensis and V. annandalei are only found 
in deeper water than has been investigated in any other Japanese lake. The only 
other Japanese species of Valvata as yet known (F. japonica, v. Martens) was des- 
cribed from a single specimen taken, presumably near the margin, in a still deeper 
lake, that of Hakone, and the type had probably strayed from deep water. Rela- 
tives of these species are all from northern latitudes. 
The remaining six species endemic in Lake Biwa are all fairly large and conspicu- 
ous forms and, with the possible exception of Melania biwae, are abundant at suit- 
able spots in the lake. The three species of Melania are closely related and seem to 
represent a strictly localized group. Pilsbry ' has pointed out that a form closely 
resembling M . multigranosa superficially (M. reiniana var. hidachiensis , Pilsbry) has 
probably been derived from another section of the genus by parallel evolution in 
another part of the Main Island of Japan. 
Nodularia biwae and Corbicula viola are fairly distinct species, but A^. reiniana 
is closely related to an otherwise isolated form {N . hirasei, Haas) the type of which 
was found in the same part of Japan but not in a lake. Haas expressed the opinion 
that N . reiniana was possibly no more than a lacustrine phase of A^. hirasei, but the 
two occur in Lake Biwa and there is no very definite evidence at present that they 
affect different types of environment. 
1 Proc. Acad. Nat. Set. Phil. I/IV, p. 119, pi. Ix, fig. 2 (1902). 
