148 Record. << of the IvrJian Mv^nnv. [Vol. XIV, 



Most of these shells are specifically identical with those now found 

 living in the Inle basin and only two species {T. analoga and T. inter- 

 media) are apparently extinct. Hydrohioides turrita, orioinally described 

 from the Irrawaddi, was not found living in the district, while the race 

 of Planorbis saigonensis that survives in the Inle Lake has changed so 

 considerably that it has to be descril^ed as a new species. The shells of 

 Succinea, Limnaea and Hydrohioides from these deposits also differ, but 

 in a less degree, from those of the living forms. 



The extinct forms have not been found elsewhere. Among the 

 living forms there is less apparent trace of the Far Eastern element noted 

 among the living mollusca of the district, but the endemic element is 

 as clearly marked, represented by the same genera {Hydrohioides and 

 Taia) and by 7 species and subspecies out of 14. We have, however, 

 in these fossil and subfossil forms a much less complete record of the 

 fauna than in the case of the living Mollusca. 



3, Conclusions. 



The geographical conclusions to be drawn from a study of the living 

 and extinct aquatic mollusca of the district, so far as the latter are known, 

 are as follows : — 



The fauna of the Inle basin has been isolated for a considerable period 

 from that of districts outside the limits of the Shan plateau, but not 

 sufficiently long for the evolution of highly specialized genera. 

 Some of the living molluscs may possibly be descended from forms more 

 peculiar than themselves that inhabited large lakes now no longer 

 existing, but the ancestors of the majority probably came from the 

 east of the Shan States, i.e., the country now watered by the Mekong 

 and the Upper Salween. We know very little about the aquatic 

 molluscs of other parts of the Shan Plateau or of the Upper Salween, 

 but, except in so far as purely lacustrine species are concerned, there is 

 no reason to think that there is any great divergence in different parts 

 of the plateau. The small Palaearctic element may have been intro- 

 duced by the agency of water-birds, which migrate annually from higher 

 latitudes. 



Part IV.— PLASTICITY AND EVOLUTION. 



We have now considered the molluscan fauna, living and extinct, 

 of the Inle Lake and the neighbouring waters from a geographical, a 

 palaeontological and a taxonomic point of view ; there remains to dis- 

 cuss it in reference to its variability, plasticity and evolution. To do 

 this it will be necessary to recapitulate the information available as to 

 each genus and species, searching out parallel instances where possible, 

 and then to summarize the whole. 



PULMONATA. 



The genus Limnaea provides us with interesting evidence as to the 

 course evolution has taken, and is taking, on the Shan Plateau so far as 



