1 92 1.] Manipur Molluscs. 531 



(2) the scarcity- of molluscan life, to which, however, there are 

 exceptions at certain places. These featuies may be observed in 

 the fauna of the liill-streams of Manipur as clearly as in that of the 

 streams of Sikkim or southern China, but as the main character of 

 the Mollusca is negative, the peculiarities can naturally be discussed 

 more appropriately when dealing with the fish than when describ- 

 ing this group. 



The Manipur valley is aflat swampy plain lying 2,600 feet above 

 , sea-level and 50 miles long by 25 miles 



anipur a ey. broad. It is surrounded by mountains consi- 



derably higher than itself and is thus completely isolated from the 

 rest of Assam, to which the Manipur State is attached politically. 

 The river system has no connection with that of the Brahmaputra, 

 but drains through a narrow pass into the Chindwin, the largest 

 tributary of the Irrawadi. 



The climate is comparatively temperate and equable. The 

 Iiif^hest and lowest shade temperatures recorded in Imphal, 

 the capital, which is situated in the central part of the valley, are 

 92° and 30° Fahr. The rainfall is moderate as compared with that 

 of some parts of Assam, but varies greatly from year to year. 

 The average at Imphal is about 70 inches, but while in 1896-97 it 

 was only 57 inches, in 1899-1900 it was over a hundred inches. The 

 winter and early spring are usually dry, but in 1920 a considerable 

 amcJunt of rain fell at the beginning of March. About half the 

 annual rainfall normally takes place between June and August 

 inclusive. The prevailing winds are from the south and the west. 

 In February and March a strong westerly breeze, apparently 

 originating in the ranges of mountains that separate the Manipur 

 from the Sjdhet valley, almost invariably arises about 10 a.m. and 

 blows until the evening, putting a stop to all fishery operations and 

 transport by boat in the open part of the larger swamps, except at 

 night and in the early morning and evening. 



The greater part of the valley is cultivated and is very fertile, 

 rice being the principal crop, while even the hill-slopes are also 

 utilized in agriculture by the Naga tribes, who burn down the 

 jungle in patclies, which thej' use for one year only. On a few of 

 the smaller hills that crop up like islands in the valley there is 

 fairly high and dense jungle, but the vegetation even here is not 

 so luxuriant as perhaps might be expected, the soil being extremely 

 friable and apparently incapable of supporting large woods or a 

 great profusion of creepers, bamboos being frequently the dominant 

 form of plant-life. 



The river-system of the valley is derived mainly from small 

 streams that arise in the Naga Hills in the 

 The River^sy^sum of the northern part of the State. These flow down 

 in an almost straight course. A few small 

 streams also come from the western slopes, but they are of no great 

 importance. No water reaches the valley from Burma and none 

 from the Sylhet cr Brahmaputra Valley. 



The whole of the basin is covered by a net-work of water- 



