68 
ZOOLOGY OF THE FAR EAST. 
The dee|> water fauna of the lakes of Switzerland and the surrounding countries, 
thanks largely in the first instance to the work of Forel, is well known; it has been 
discussed recently in great detail by Zschokke in his book Die 
Deep-water Mollusca of 7^ i , • ,7-7/ 
European lakes Tiefsec Faufia 0,67 Seen Mttteieuropas , eine geographisch-jaun- 
istische Studie (Leipzig, 1911), from which the following facts 
are taken. 
Twelve ' ' species ' ' of Gastropoda and twenty- three of Lamellibranchiata have 
been recorded from depths of 20 metres and over, all the gastropods and the majority 
of the bivalves coming from at least 50 metres. The lakes are deeper than L. Biwa 
and representatives of both groups occur as deep as or deeper than 260 metres. 
The Gastropoda belong, with two exceptions, to genera also found in the Japan- 
ese lake. The exceptions are Pyrgula and Neritella : the common genera are Lim- 
naea, Bithynia, Vivipara and Valvata. Most of these are represented in deep 
water by single species, but there are three deep-water forms of Limitaea and five 
of Valvata. How far all these forms are to be regarded as distinct species is very 
doubtful. 
The three forms of Limnaea occur between 50 and 260 metres, the single Vivipara 
at 60 metres and the five forms of Valvata between 60 and 200 metres ; the Bithynia is 
found at 60 metres. Most of the " species " are restricted to deep water. 
In Asiatic lakes generally, so far as my experience goes, species of Limnaea are 
rarely true lacustrine forms, though they often occur in abundance in pools, ditches 
and backwaters; while Bithynia is found only at the margin among stones. The 
absence of these two genera from the deep-water fauna of Lake Biwa depends, there- 
fore, in all probability on factors other than bathymetric. It is noteworthy that 
the only gastropods that occur in that lake below 40 fathoms are either intrusive 
forms from shallow water, which are apparently not affected so far as size is con- 
cerned, or else belong to Valvata, the genus perhaps best represented in the European 
deep-water lacustrine fauna. Though the deep-water species from the two conti- 
nents are not closely related, the peculiarities of the shells are the same. 
Turning to the Lamellibranchiata of deep water in European lakes, the only 
genus represented (except for possible stray immigrations of individual Unionidae) is 
Pisidium, of which, as has already been pointed out, no less than 23 so-called species 
have been recorded from depths between 20 and 300 metres In dealing with P. 
casertanum I have said about these all that need be said. 
Of course all the deep-water Mollusca of central Europe have not been found in 
any one lake, and further investigations in other Japanese lakes, among which Lake 
Biwa ranks only fifth in point of depth, are necessary before a proper comparison 
can be made. It can, however, already be claimed that, allowing for differences in 
the fauna of two regions so remote from one another, the deep-water lacustrine Mol- 
lusca of the temperate Far East do not appear to differ materially in general charac- 
ters from those of the lakes of central Europe, and that at any rate some of 
the genera represented in deep water in the latter region are also represented 
at similar depths in Japan. So far as Japan is concerned, there is also evidence 
