1919.] N. ANNANDALE & B. PrasHap: Gastropod Molluscs. 105 
those that now prevail in the districts where the specimens were 
collected. 
Both the species of Limnaea from Seistan and those from 
Mesopotamia have in most instances small and very thin and fragile 
shells, which are sculptured with more or less prominent longitudinal 
tibs, but the ribs on those from Mesopotamia are much more pro- 
nounced than on Seistan shells. This is so even in identical 
species. The shape of the shell in most forms exhibits slightly 
different modifications in the two countries. The two Seistan 
species are both rather narrow but have relatively large and 
patent mouths and short, pointed spires. The three distinct Meso- 
potamian forms exhibit a tendency, on the other hand, to grow 
broad in proportion to their height, while their spires are still 
shorter. ‘Their mouths, in the broader forms, are relatively narrow 
but on the whole resemble those of L. gedrosiana and L. bactriana. 
An entirely new character appears in Mesopotamia in the canaliza- 
tion of the suture above the body-whorl found in all the endemic 
species and well-defined races. 
How far can these differences and resemblances be correlated 
with similar peculiarities of the shell from different or identical 
types of environment in other countries ? Baker,’ in his fine mono- 
graph of the North American Limnaeidae, states that shells from 
saline districts in that country are prominently ribbed, but whether 
this ribbing is associated with the presence of any particular salt 
we are not informed. We know ina general way what salts are 
present in the waters of Seistan,? but no information is available 
about the composition of those of the Euphratic delta. 
Thinness, fragility and paleness of shell in Lemmaca are usually 
associated with unfavourable conditions of life, but more precise 
information is needed on the point. The reduction of the spire and 
enlargement of the body-whorl and mouth provide additional space 
for the branchial chamber and allow a larger portion of the animal 
to be extended from the shell. This character, moreover, seems 
to be definitely correlated with difficulty in obtaining the necessary 
oxygen, and we find different forms of the same species 3 (living in 
different types of environment in the same localities) in which it is 
more developed in those that live in still water than in those that 
inhabit streams. Both in Seistan and in Mesopotamia the Lamnaeae 
live mainly in flooded country, perishing in large numbers when the 
floods subside. The few individuals that survive to perpetuate the 
species from year to year retire to shallow pools, backwaters or 
lakes. In the course of such an existence conditions must be 
encountered in which life is difficult and water lacking in free 
oxygen. 
The Limnaeae of Mesopotamia are, except L. peregra canalifera, 
clearly depauperated forms, and the considerably larger size attained 
{ Baker, Spec. Pub. Chicago Ac. Sci. U11 (1911). 
i Annandale, Rec. Ind. Mus. XVIII, pp. 10, 15 (1919). 
8 Annandale, Rec. Ind. Mus. XIV, p. 149, fig. 4 (1918). 
