158 



UecorcU of the. Indian Museum. fVoL. XI V, 



Fig. 



7._Shells of Melania 



baccata ( x §). 

 . Shell from Hsipaw 



ill the Northern 



8han States. 

 . Shell of small ])hase 



of var. eluiKjala 



from the Yawii- 



ghwe river. 



M. haccata is a species or a group of species that raises great difficulties 

 in nomenclature owing to the variability and plastic character of the 

 shell. It is found chiefly in the Shan Plateau, but in different parts 

 thereof a number of local races seem to have become differentiated. 

 All the shells, both recent and fossil, that we obtained in the two basins 

 belono' in a sense to a single race for which it has been necessary to 

 find a new name. In reference to the elongation of the shell I have 



called it " subspecies elongata." But this 

 race has at least two, if not three, phases, 

 and shells from the same environment exhibit 

 considerable individual variation in sculp- 

 ture. The shells of the different phases do 

 not differ in shape, but those from streams 

 both in the Inle and the He-Ho plains are 

 very considerably smaller than those from a 

 swamp in the latter. Subfossil shells from 

 both peaty and calcareous deposits on the 

 He-Ho plain seem to have the sculpture 

 rather more sharply developed than those 

 from streams, with which they agree in size. 

 In a large series of living specimens from the 

 Yawnghwe river the majority have three 

 well-developed spiral ridges with series of 

 tubercles on the last two whorls, but in a 

 small proportion of specimens there are only 

 two ridges of the kind. No actually inter- 

 mediate individuals were found, but in one or two shells the uppermost 

 of the simple ridges below the series of tubercles on the body-whorl has 

 a slightly tubercular character. The same variation occurs among sub- 

 fossil shells. Further information is necessary before we can discuss in 

 detail the meaning of the differences in this species. I figure in outline 

 a specimen of a shell from the Northern Shan States side by side with 

 one of the smaller phase of the sub-species elongata for comparison. 



I have recognized the genus Hydrohioides as consisting of five species, 

 one of which is very unlike the others in appearance. This species 

 {H. fhijscus) departs widely in form of the shell from the type usual in 

 Bithynia, the genus from w^hich Hydrohioides is derived, and is not 

 plastic. Specimens from a swamp near He-Ho hardly differ from those 

 taken in the central region of the Inle Lake except in being a little smaller. 

 This species is only known from the two basins. Three other species of 

 more normal shape are also, so far as we know, neither variable nor 

 plastic. 



It is otherwise, however, with the remaining species of the genus — 

 H. nassa, which has a wide range in the Shan States and departs much 

 less far from Bithynia than does H. physcus, but further than H. avarix, 

 the species to which it is most closely related. H. nassa was originally 

 described from a locality that lies some considerable distance east or 

 north-east of the Inle Lake, but is common all over Yawnghwe 

 and the neighbouring states. There are cotypes of the species in the 

 collection of the Indian Museum. They differ very slightly from the 



