I91S.] N. Annandale : Molluscs of Ike Iitle Lake. 1'73 



loses the power of absorbing the ridge left when the shell grows beyond 

 the old lip. All this is in favour of the view that the varix is a vestigial 

 structure, vestigial that is to say so far as the individual and not the 

 race is concerned. It may otherwise be considered as a sign of the 

 approach of senescence or at any rate of full maturity, for when the old 

 lip ceases to be absorbed the animal has not the power to grow very 

 much larger. 



It is difficult to see in what way differences of size in the shells of 

 Melania or of shape and sculpture in those of Taia could be of any benefit 

 to the race either directly or indirectly. The shells in the latter genus 

 are as a rule so thin and fragile, even when highly decorated, that they 

 could not protect the animal from powerful enemies of any kind. 

 Neither could the knobs and scales on their surface protect them from 

 parasites. T. intha at any rate, the species which I have been able 

 to observe most closely in natural conditions, seems to be a peculiarly 

 sluggish and unprotected animal, and to be altogether devoid of enemies 

 except leeches of the genus Gloss osiphonia, which make their way in 

 through the mouth of the shell and are not deterred from doing so by 

 the sculpture round the aperture or on the surface. 



(4) 



We have seen that in Limnaea the cause of change in the shape of 

 the shell is, in some species, the change from running to still water or 

 conversely, and that this change acts directly on the young individual. 

 In other species of the same genus, however, other forces come into 

 play and the case becomes much more complicated. Indeed, almost 

 all that can be said with certainty is that the shells of individuals living 

 in water of peculiar chemical composition, as all the Inle species 

 do, are usually dwarfed, and that individuals living in deep water 

 are still more strongly dwarfed and very narrow and have the mouth 

 of the shell very long and the spire short, and that these are essentially 

 immature characters. 



In Melania tuherculata overcrowding, especially in a small space, 

 and also undue salinity of the water in some cases, produces dwarfing, 

 but in the Lake of Tiberias, where the water is distinctly saline, the 

 shells are large and well-developed. 



We know that Hijclrobioides nassa distoma survived for a considerable 

 period both in lacustrine and paludine conditions without change, al- 

 though the species, probably under slightly different conditions, has 

 developed a distinct race in the central region of the Inle Lake, and the 

 race distoma has become extinct. 



Some of the different races of Taia that have become so far differ- 

 entiated as to be regarded conventionally as distinct species, have been 

 produced, so far as we can see, in circumstances that differ little, and the 

 same thing has apparently occurred in Margarya in Yunnan, in the 

 Tertiary Viviparidae of Slavonia and in other widely scattered instances. 



It follows almost as a corollary that similar modifications may be 

 brought about in very different biological circumstances. This is 

 clearly shown by the resemblance between the shells of Succinea indica 

 and Limnaea mimetica ; but it must be noted that only the Limnaea can 



