66 
ZOOLOGY OF THE FAR EAST. 
which is even deeper than I^ake Biwa. No particulars are available as to the depth 
at which it lives, but even if it were taken at the edge, as is most probable, it may 
have strayed from deep water ; it appears to have been captured alive. In Lake 
Biwa I found a single dead shell of Valvata hiwaensis embedded in a sponge growing 
within a few yards off the shore. V . japonica is stated by von Martens ' (who des- 
cribed it in 1877) to be' intermediate between the European V. naticina and the 
North American V. sincera; it is probably related closely to the Biwa forms. 
The Japanese representatives of this genus may thus be regarded as forms of 
northern origin. A few species of Valvata may possibly make their way into the 
tropical parts of Asia, but the only form recorded (with a query as to the genus) 
from India south of the Himalayas {Valvata? microscopica, Nevill)* belongs to the 
marine family Cyclostrematidae.' The genus, indeed, is perhaps exclusively Holarctic. 
The Pisidium found in deep water in Lake Biwa belongs, as is stated above, to 
the Palaearctic species P. casertanum (Poli). Both shallow-water and comparatively 
deep-water forms from Lake Baikal are relegated to the synonomy of P. casertanumhy 
Woodward, namely P. maculatum and P. trigonoides , Dybowski* (from 20 to 60 metres) 
and P. dubium, Lindholm ^ (from 6 to 8 metres). To judge from the published figures 
of these forms, Biwa shells resemble P. dubium very closely in lateral outline, but are 
a little smaller and slightly more compressed ; they are more elongate and have the 
umbones less prominent than either of Dybowski 's "species," which, though he 
places them in different subgenera, are perhaps mere growth-stages of a single phase. 
European specimens of P. casertanum are usually larger than my Japanese shells, 
but, even in the lacustrine phase, exhibit great variation both in size and shape. 
Among the 23 nominal species of Pisidium listed by Zschokke ^ 
Deep-water forms of Pisidium. ••^111 r ji-r-v , r 
as occurrmg m the lakes of central Europe m water of 30 
metres or over, only one (P. italicum, Clessin) is included in the synonomy of P. caser- 
tanum by Woodward, who regards most of the deep-water forms of the genus as of 
doubtful status but suggests that many are probably forms of P. pusillum (Gmelin). 
Moreover, it is doubtful whether the P. italicum from deep water (Lago Maggiore in 
northern Ital}'^; 80 m.) is absolutely identical with the shallow-water form to which 
that name has been applied. Zschokke, following Clessin and Forel, classifies two of 
the twenty-three deep-water species as Abkommlinge von Pisidium italicum" 
(namely P. luganense and P. locarnense) , and six as allied to P. fossarinum, which ac- 
cording to Woodward is synonymous at any rate in part with P. casertanum. It 
would be a mere waste of time to attempt to discuss the minute and often inconstant 
differences between all these forms. Indeed, it is sufficient for the present to say 
that Japanese deep-water shells of P. casertanum agree in their small size and elong- 
1 von Martens, Sb. Ges. nat. Freunde, p. ii6 (1877). 
^ Nevill, Hand List Moll. Ind. Mus. II, p. 17 (1884). 
» Annandale and Kemp, Mem. Ind. Mus. V, p. 347, fig. 3 (1916). 
* See Kobelt in Rossmassler's Icon. Land-tmd Susswasser-Moll. (new ed.) X, pp. 32, 33, pi. cclxxix, fig. 1807; pi. 
cclxxxi, fig. 1809 (1903). 
6 In Korotneff's IViss. Ergehn. Zool. Exp. Baikal-See IV {Mollusken), p. 83, pi. ii, figs. 45, 46 (1909). 
6 Zschokke, Die Tiefsee Fauna d.See Mittelenropas, p. 157 (I^eipzig, 1911). 
