-•3U LAND AND FKESHWATER 



the well-worked area of the Garo, Khasi, and Jaintia Hills ? I may 

 mention a few of these : Macrovlilamys richihiensis, sathilaensis, and 

 zemoensis, with a free caecum, from which we may conclude that 

 species with the typical coil have been derived ; DuUnr/ia,Staffordia, 

 and a slug-like form described as Tcstaceila"^ dil-rangensis, G.-A.*, a 

 most pecuhar shell, animal not known, but certainly not a TestaceUa. 

 That remarkable slug Anadenus is a common species in Sikkim and 

 Nepal, and has spread westward to Gurhwal and Simla Hills, just 

 as Alcceus and Dijjlommatlna are represented there bj' only two or 

 three species. Anadenus no doubt extends throughout the extent 

 of the as yet unworked Bhutan Himalaya, for I have lately recorded 

 from C ilcutta a line new species found in the hills of North-east 

 Munipur. 



The extent and direction of this old land has to be traced, but it 

 apjjears to lie in an eastern direction, several species being common 

 to Eastern Assam and Upper Burmah. The axis of elevation of 

 the Khasi Hills is comparatively recent post-Tertiary; its molluscan 

 fauna could easily have been derived from the north and east and 

 become modified. 



If we consider the generally accepted zoological regions, there 

 are conditions and forces affecting the distribution of the Mollusca 

 which do not apply to the vertebrate and many invertebrate 

 animals — such as dispersal on the lines of main drainage. 



A glance at the map of India, on longitude 84° E., shows two 

 great rivers, the Gandak and Xosi, having their sources in the main 

 range of the Himalaya and joining the Ganges just above Eajmalial, 

 where that river washes the base of those hills. Further to the 

 east is the Teesta draining Sikkim, the Am Mochu or Toorsa, the 

 Gudhadhur and Manass the mountains of Bhutan, with many con- 

 siderable intermediate rivers. The outer ranges through which these 

 rivers flow are clothed with a dense forest-growth, often down to 

 the water's edge, which continues for many miles after the rivers 

 leave the mountains to flow through what is called the Terai ; and 

 beyond this, where the great grass-plains begin and the cultivated 

 area is entered, many of the M'atercourses have their banks fringed 

 with forest, which, before the advent of Man on the scene and his 

 annual burning of the grass, no doubt covered a far larger area of 

 the plain country than it does now. 



It follows that there has been a natural transport of the 

 Mollusca inhabiting the mountain country southward. "With the 

 rise of the rivers during the rains thousands must be washed 

 off the banks. Snails are not killed immediately by immersion in 

 fresh water ; they might be carried far down, be washed up on the 

 bank, and finally crawl away into safety in the adjoining jungle- 

 growth. Besides, there are many ways in which they might be 

 carried even further, inside bamboos or on floating trees. 



I think it is safe to consider that the molluscan fauna of the 



great delta of the Ganges and Brahmaputra has had its origin in 



the Himalayan slopes. Several Calcutta species and those of the 



Kajmahal Hills find their nearest allies anatomically in species 



* J. A. S. B. vol. xlv. pt. 2(1876). 



