1919. ] N. ANNANDALE & B. PrasHaD: Mollusca. 25 
chemical stimulus with the facility sometimes associated with this 
factor in their environment, and are not particularly plastic or 
variable or at all exuberant in shell-sculpture. This is probably 
because other conditions are unfavourable—extremes of tempera- 
ture, drought and lack of food—and the struggle for existence is 
too keen. The composition of the water has, indeed, had one 
effect, physical rather than biological, in preserving such sculp- 
ture as the shells possess intact from erosion. Probably it has 
acted indirectly, by discouraging the growth of corroding algae. 
But, even so, the Limnaeae do not develop the strong longitudi- 
nal ribs formed on the shells of those that live in saline waters 
in North America.! 
The shells of all the species of Gastropods are small and in 
most cases very thin. They are of perfect form, neither distorted 
nor abnormal in other respects. Their colours, both of shell and 
soft parts, are pale. They constitute, in fact, alimited but very 
normal paludine fauna such as might be found in any temperate 
region. 
No peculiar lacustrine species or even well-marked phases have 
as yet been evolved in the Hamun-i-Helmand. 
In Seistan the recent geological history of the country has 
been of such a nature that subfossil shells are extremely abun- 
dant nearly everywhere except in the central parts of old lake- 
basins, while owing to the annual floods enormous numbers of 
quite recent shells are scattered over the country. In the deposits, 
both recent and historically ancient, examined both in the 
northern and the southern districts only two species. were found 
(Limnaea hordeum, evidently a scarce form, and Segmentina cala- 
thus, a widely distributed but somewhat sporadic one) which were 
not found living in the country. The absence of Melantidae from 
these deposits was a noteworthy feature. The commonest species 
in them at most places were Amnicola sistanica, Limnaea gedro- 
siana, Gyraulus convexiusculus, G. euphraticus and Corbicula 
fuminalis. At some spots, evidently those reached with fair 
regularity by recent floods, Lamellidens marginalis was also 
present in large numbers, and at one place Vivipara hilmandensis 
was common. 
An interesting question in the bionomics of freshwater mol- 
lusecs in a country like those under consideration is that of hiber- 
nation and aestivation and their effect on sexual activity. We 
give evidence below that certain species (mainly those of the 
genera Melanoides, Corbicula and Lamellidens) burrow into mud 
or sand either at the approach of winter or on the sinking of the 
annual floods. Perhaps this is also true of Amnicola ststanica. 
The Limnaeidae and Planorbidae, however, remain active through- 
out the winter. In Seistan and northern Baluchistan, and also at 
certain places in the North West Frontier Province, the eggs of 
ae Baker, Chicago Academy of Sciences, special publication 3, p. 30 
IQI1). 
