1918.] 



N. Annandale : Molhisrs of the InU Lake. 



147 



No exclusively Indian species is on the list, but Melania terehra 

 has been known hitherto only from Assam, and Succinea indica from 

 the Himalayas. 



The living- molluscan fauna of the Inle and He-Ho basins is, there- 

 fore, to a considerable extent endemic, so far as species are concerned. 

 A Far Eastern element, represented by one genus and by about one- 

 seventh of the species, is also apparent. A certain proportion of the 

 species, as well as the great majority of the genera, are of wide geo- 

 graphical range, while two of the species are Palaearctic or have strong 

 Palaearctic affinities. Most of the characteristic genera of Indo-China 

 {e.g., Lacunopsis, Julliena, etc.) are, however, absent and the Palae- 

 arctic forms are both Eastern. It seems by no means improbable, 

 therefore, that the Far Eastern element has immigrated on to the 

 plateau from the north-west, but the importance of the absence of such 

 genera as Lacunopsis and Julliena is somewhat decreased by the fact 

 that they are mainly fluviatile, vfhereas the fauna with which we are 

 dealing is predominantly lacustrine. The Palaearctic forms, moreover, 

 may have been introduced by migratory water-birds such as ducks, and 

 have been able to survive owing to the comparatively low temperature 

 of the plateau. We know that one of them has been established in the 

 neighbourhood for a considerable period. 



2. Fossil and Subfossil Mollusca. 



I have already discussed the different deposits in the Inle and the 

 He-Ho basins from which shells of aquatic Mollusca have been obtained. 

 Here it is only necessary to consider the shells in two categories, those of 

 fossil and of subfossil forms. The distinction is of course conventional. 



The fossil forms were found in two beds, in cave-deposits near 

 Yawnghwe in a basin probably at one time subsidiary to the Inle basin, 

 and in an old lake-deposit in the He-Ho plain. In these two beds the 

 following species and subspecies occur : — 



Limnaea, ? sp. nov. 

 Limnaea shanen.sis. 

 Planorbis trochoideus. 

 Melania variabilis. 

 Uydrobioides nassa distoma. 



Tain theobaldi. 



Taia obesa. 



Taia conicn. 



Taia cylindrica. 



Taia lacuslris. 



Of these 10 forms, 6 (5 species and 1 subspecies) are apparently 

 extinct and have not been found elsewhere. Moreover, of the four 

 surviving species only Planorbis conoideus and Taia theobaldi survive 

 in identical form, the shells of Limnaea shanensis and the Melania 

 (which we did not find living in either basin) being slightlv different 

 from those of living individuals. 



The subfossil shells were taken in superficial deposits on the He-Ho 

 plain. The following is a list of the species and subspecies : — 



Succinea indica. 

 Limnaea shanensis. 

 Planorbis exustus. 

 Planorbis sair/onensis. 

 Planorbis trochoideus. 

 Ulelania tuberculata. 

 Melania haccata eloiu/ata. 



Ilydroh ioides f u rritn . 



Uydrobioides nassa distorna. 



Amnicoki alllcohi. 



Taia interwedia. 



Taia theobaldi. 



Taia analoga. 



Corbicida noetlinr/i. 



