Mollusca of Lake Biwa. 
53 
Pisidium casertanum (Poli). 
(PI. Ill, fig. 14.) 
1909. Pisidium (Fluminina) dubium, Lindholm in Korotneff's Wiss. Ergehn. Zool. Exp. 
Baikal-See, IV (Mollusken), p. 85, pi. ii, figs. 45, 46. 
1916. Pisidium casertanum form lacustris, Preston, Ann. Mag. Nat. Hist. (8) XVII, p. 162. 
Shells from Lake Biwa are referred by Preston, apparently in consultation with 
B. B. Woodward, to the " forma lacustris," but are said by the latter to be rather 
more oval than usual. I take it that the ''forma lacustris " is the phase mentioned 
by Woodward {op. cii., p. 36) as "a. lake or still water form, which almost 
amounts to a variety." This phase is characterized as being rounder than the type 
and more compressed and having the hinge, which is narrower and lighter, less arcuate 
and with a less pronounced flexure. 
Of all the numerous phases and races of P. casertanum described by various 
authors and not regarded by Woodward as even on a level with his "lake or still 
water form,' ' shells from L- Biwa resemble most closely those from shallow water (one 
fath.) in L- Baikal to which Lindholm gave the name P. dubium. They differ only 
in being a little smaller and a trifle more compressed, and probably in greater fragility 
and translucency — all characters commonly associated with life in deep water. 
The following tables illustrate the resemblances and differences so far as measure- 
ments (given in millemetres) and proportions can. 
Deep-water Biwa Phase. Shallow-water Baikal Phase (dubium). 
Length 4-8. 3 6. 3-9. 3-9. 4-0. 4*04. 6-o. 5-9. 5-0. 
Height 3'2. 3-1. 3-0. 3-2. 3 3. 3-2. 4*8. 4'8- 4"0- 
Thickness 2-06. 1-9. i-8. 1-9. 2-0. 2*06. 3'2. 3-1. 27. 
Height to length (average) i : 21 i : 24. 
Thickness to height i : 1-64 i : i"5i- 
Thickness to length 1:2 i : i-88. 
Two other ''species" from L. Baikal (P. maculatum and P. trigonoides , Dybows- 
ki) ' are considered by Woodward as synonymous with P. casertanum. They were 
both found in 20-60 fathoms and differ notably from P. dubium in their prominent 
umbones, a feature noted by Clessin' in deep-water forms of the genus from Swiss 
lakes but conspicuously absent from the deep-water Japanese specimens. Though 
Dybowski and Kobelt place these two Siberian forms in different subgenera (P. macu- 
latum in Fluminina and P. trigonoides in Fossarina) it is by no means certain that 
they are more than growth-stages of a single form. 
P. casertanum is very abundant in I^. Biwa at depths greater than about 17 
fathoms. It usually occurs on a bare muddy bottom and is found with Valvata 
biwaensis and V. annandalei. L. Baikal is the only other Asiatic locahty from 
which any phase of the species has yet been recorded, but it is wide-spread in Europe. 
I See Kobelt in the new edition of Rossmassler's Icoyi. Land-und Sitsswasser Moll. X, pp. 32, 33, pi. cclxxix, fig. 
1807; pi. cclxxxi, fig. 1809 (1903)- 
•2 Bull. Soc. Vaudoise Nat. Sci. XIV, p. 240, pi. iii (1877). 
