52 
ZOOLOGY OF THE FAR EAST. 
The typical form is found in Lake Biwa at depths of from about 2 to 25 fathoms, 
as a rule on a sandy bottom. In the deeper part of the lake, on a muddy bottom in 
about 43 fathoms, I took a few specimens of what appears to be a dwarfed form. 
The largest shell is 11 mm. long, 11 mm. high, and 8 mm. thick with the two valves 
together. It is both relatively shorter and more globose, as well as being more sym- 
metrical than the typical form ; the hinge-structure is identical. I consider these 
shells dwarfed and not merely young because they are relatively shorter, as well as 
being thicker, than young shells of the typical form. The colour of the inner surface 
is also darker. Externally the shells are pale olivaceous green stained with a darker 
shade and with obscure narrow vertical rays of a reddish tinge. 
A very peculiar polyzoon of the genus Paludicella is often found growing on the 
posterior end of the shells of this species, while the sponge Spongilla dementis not in- 
frequently settles on the same part of the shell and sometimes suffocates the animal 
by its growth. 
C. viola is only known from Lake Biwa. 
Genus Sphaerium, Scopoli. 
1901. Sphaerium, Pilsbry, Proc. Acad. Nat. Set. Phil. LIU, p. 406. 
According to Pilsbry, all the Japanese species of this genus, of which only four 
are at present known, belong to the subgenus Calyculina. 
Sphaerium heterodon, Pilsbry. 
(PI. Ill, fig. 13.) 
igoi. sphaerium heterodon, Pilsbry, op. cit., p. 406. 
I have not been able to refer to the original description of this species in Pilsbry 's 
Cat. Mar. Moll. Jap., but from the same author's remarks in the paper quoted above 
and from the figure given in the pamphlet on the Mollusca of Lake Biwa published 
by the Shiga Fishery Department, I take the form commonly found in the district to 
be S. heteorodon. 
I did not find any specimens of Sphaerium in Lake Biwa, but obtained a fair 
series from ditches in the fishery station at Hikone. The species was orginally des- 
cribed from the northern part of Kiushu, the most southerly of the larger islands of 
Japan proper. It has not, so far as I am able to say, been found further north than 
Lake Biwa. 
Genus Pisidium, Pfeiffer. 
1913. ' Pisidium, B. B. Woodward, Cat. Brit. Pisidium Brit. Mus., p. 31. 
Only two species of this Holarctic genus have as yet been found in Japan, namely 
P. japonicum,' Pils. & Hir. from a lake in Hokkaido and P. casertanum (PoH), a widely 
distributed Palaearctic species of which I obtained specimens in Lake Biwa. 
Proc. Acad Nat. Set. Phil. LX, p. 35, fig. i (1908). 
