Mollusca of the Tai-Hu. 
303 
and the scarcity of food due to poverty of vegetation. Another factor that may be 
considered is that of overcrowding. I have noticed in India that in ponds in which 
a few species are present in great abundance the individuals of these species are often 
dwarfed, even when individuals of other species more sparingly represented attain a 
normal size. It is noteworthy, therefore, that almost' all the Tai-Hu molluscs except 
the large Vivipara are extremely abundant in the lake. We do not yet know, how- 
ever, whether overcrowding acts directly on the individual through the products of 
metabolism, or whether it does not rather imply an intensive reproduction due 
to other unfavourable circumstances such as scarcity of food or oxygen. 
It may further be noted here that one species [Pseudovivipara hypocrites) is 
blind, while another {Hypsohia minuscula) has very small eyes — facts probably cor- 
related with the muddy water in which they live. 
The estuarine element in the Tai-Hu fauna, an element by no means confined to the 
Mollusca, is represented among the Gastropoda by species of the genera Stenothyra and 
Assiminea and among the bivalves by a species of Modiola. It is probable that all 
these species make their way much higher up the river- system, but only the Modiola 
is known to do so. In India Stenothyra, Assiminea and Modiola are all characteristic 
of the upper estuaries of the larger rivers, but I do not know of any species that has 
established itself in permanently fresh water. A . francesiae, Gray, which resembles 
A . scalaris in its quasi-aquatic habits, has made its way up the Hughli at least as far 
as tidal limits, that is to say for over a hundred miles from the open sea, and is com- 
mon not only on the banks of the river but also in flooded fields in the vicinity, but I 
have never seen any species of either Stenothyra or Modiola, both of which are strictly 
aquatic, further inland than Calcutta, in the neighbourhood of which there is much 
brackish water. The Tai-Hu is not so far inland as Calcutta, but it is not connected 
with any tidal water-way. Its water, moireover, is not liable to admixture of salt, 
for the fresh water that pours down in such vast quantities through the mouths of the 
Yangtse has more influence in counteracting the tides than that of any of the Gangetic 
effluents and brackish water apparently extends inland for a much shorter distance 
in the Chinese than in the Indian delta ; indeed, at the mouth of the Yangste practi- 
cally fresh water extends for a considerable distance out to sea. 
We know as yet very little about the estuarine fauna of China, but the occurrence 
in the Tai-Hu of estuarine species and genera proves at any rate that the well-known 
estuarine element present in the fluviatile fauna of the Ganges has its parallel 
in China. 
The molluscs of the Tai-Hu do not cast much light of a geographical nature 
on the aquatic fauna of China. The genus Hypsohia has, however, some interest from 
this point of view. It was hitherto known from two species, H. humida from the 
Upper Yangtse and H. nosophora from an inland district of the Main Island of Japan, 
but these species have been placed in separate genera and have peculiar habits, being 
only quasi-aquatic. "The discovery of a third, completely aquatic species {H. minus- 
cula) in the Yangtse delta, therefore, provides a record of intermediate locaUty for 
the genus. The new Hydrobiid genus Pseudovivipara probably occurs in Hainan as 
