104 Records of the Indian Museum. [VoL. XVIII, 
among the Pulmonata at any rate no description unaccompanied 
by a recognizable figure should be considered valid. 
Genus Limnaea, Lamarck. 
The species that occur in Lower Mesopotamia are of con- 
siderable interest. They fall into two of the main groups of the 
genus, namely those that may be called, in quite a general sense, 
those of L. auricularia and L. truncatula. Most of them we assign 
to the former group. These species are all variable and at first 
sight it might seem that most of those of southern Persia and the 
lower valley of the Tigris and Euphrates were conspecific, merely 
separable into numerous races and varieties. A careful study ofa 
large amount of material has, however, convinced us that this is 
not the case, and that a number of species actually exist which 
can be separated by anatomical as well as merely conchological 
characters. The examination of young shells is most important 
for this reason, and we find that adults which resemble one 
another rather closely can often be traced back to young forms that 
differ considerably. In conditions such as those found in Lower 
Mesopotamia, and also in most parts of Persia, convergence seems 
to have taken place between different species of the genus on a 
fairly extensive scale, and forms which are perhaps not closely 
related genetically have come to resemble one another owing to 
the plastic influence of environment. 
In Seistan we know of two species of the L. auricularia group 
(L. gedrosiana and L. baclrianu) which at first sight appear almost 
identical, but on a detailed investigation have proved quite dis- 
tinct. We are of the opinion that these species each had a differ- 
ent line of ancestry, but have thought it best in the present 
state of our knowledge not to discuss these lines of ancestry in 
detail. 
Similarly in collections from Tower Mesopotamia we have 
found five forms that seemed to us on a superficial examination to 
grade the one into the other. ‘Two of these (L. gedrosiana and 
L. bactriana) are the same as the two Seistan species. The others 
are L. peregra canalifera, L. tenera euphratica and L. cor. 
It is important, therefore, to consider what common features 
are to be found in the forms of each country, and to what extent 
these features can be correlated with peculiarities of environment. 
Both Lower Mesopotamia and Seistan are countries that have a 
prolonged dry season, when the temperature is high, and are 
nevertheless liable to floods. In Seistan the water always contains 
a high percentage of mineral salts, while the delta of the Tigris and 
Euphrates is an estuarine tract in which even water that is practi- 
cally fresh is probably liable to be contaminated with water of com- 
paratively high salinity. Moreover, this tract has been gradually 
extending southwards and receding from the north for a consider- 
able period, and many of our specimens are from old deposits that 
must have been laid down in conditions far more estuarine than 
