Mollusca of Lake Biwa. 67 
ate form with those of aUied types from deep water in Europe, of which Zschokke 
{op. cit.,p. 162) writes as follows: — 
"Die Tiefenpisidien erscheinen somit als kleine, schwache, unscheinbare Kum- 
merformen. Mit innen stimmen im allgemeirren Habitus, besonders in der ger- 
ingen Grobe, der Zerbrechhchkeit der Schale und in der relativ zur Lange sich 
dehnenden Korperbreite die Gastropoden der profunden Region uberein." 
There is one point, however, in which the Pisidium of Lake Biwa differs from 
its Swiss deep-water congeners namely in its comparatively small umbones. Forms 
from 20 to 60 metres in Lake Baikal agree with the Swiss forms in this respect, though 
some at least of the shallow-water forms resemble the Japanese one. 
If these are the characteristics associated with life in deep water in the case of a 
naturally thin and small-shelled genus such as Pisidium it does not follow necessarily 
that they would be developed also in shells of a more robust type. Indeed, so far as 
our scanty evidence goes, it seems that in Corbicula, of which I obtained {v. p. 52 
ante) a few dwarfed, shortened, inflated and highly-coloured 
Pccuiiadti^cs^of^g,r6rc«/a specimens in 260 feet (together with normal or approxi- 
mately normal shells of Vivipara and Melania) in Lake 
Biwa, very different results follow. This is evidently not due to an inability on 
the part of Corbicula to produce thin-shelled and colourless forms, for in the Whang- 
poo River below Shanghai, at a depth of between 6 5 and 7*5 metres, just such a 
form as might be expected on the analogy of Pisidium in deep water occurs not un- 
commonly. This is evidently Prime's Corbicula tenuistriata , which was described 
from an unknown locaHty, but neither his description* nor Reeve's figure^ does full 
justice to its very Mn-Corhicula-like appearance, with its small, palhd, thin, subtri- 
agonal shell, which to the naked eye posses.ses the most delicate transverse concentric 
striae instead of the usual ridges. The species is, nevertheless, a true Corbicula, for 
Dr. Ekendranath Ghosh, who has been kind enough to examine my specimens anato- 
mically, informs me that he can find practically no difference between the soft parts 
and those of the thick-shelled, highly-coloured C. largillierti common in still water in 
the same part of China. 
The peculiarities of C. tenuistriata, though they are similar in many points to 
those of deep-water forms of Pisidium, are clearly not due to Hfe in deep water, but 
are perhaps correlated with life in very stiff mud lying in a strong current beneath 
very muddy and therefore opaque water; possibly also with a scanty food-supply. 
It is hard to say what precise factors in deep-water existence— cold, darkness, lack of 
food, insufficient oxygen, water-pressure, etc.— exert a direct or indirect influence in 
producing special characters, and it is evident that different factors may have greater 
effect in some forms of Mollusca than in others. 
The deep-water molluscan fauna of Lake Biwa may now be compared with that 
of other lakes in Europe and Asia. 
1 SeeClessin, Bull Soc. Vaudoise Nat. Set. XIV, p. 240, pi. iii (1877). 
2 Proc. Zool. Soc. London i860, p. 322. 
3 Conch. Icon. XX (Cyrena), pi. xv, fig. 80 (1878). 
