4 Memoirs of the Indian Museum. [Vol. V, 



their bases remain under water throughout the year. In this area there are also a 

 few flatter and more sandy islands the margins of which slope gradually, but the 

 most remarkable and the largest island in the whole lake is Nalbano, which lies not 

 very far within the Mugger-Mukh. Nalbano is a great sand-bank completely covered 

 with tall reeds, the roots of which are submerged when the water is high, so that only 

 the leaves and inflorescences are visible above the surface. 



The islands of the outer channel, including Barnikuda, are also sand-banks, but 

 at most support in the way of vegetation no more than a scanty growth of short grass 

 with, in the case of Barnikuda, a few stunted shrubs. 



Generally speaking the bottom of the main area is muddy, while that of the 



outer part of the outer channel is sandy. In the former 

 its nature is so uniform, notwithstanding the admixture 

 of a certain amount of sand at some places, that the small actual differences have as 

 a rule little effect on the fauna, and it is only in the neighbourhood of Nalbano and 

 on the shores of some of the other islands that true arenicolous species occur in this 

 area. The mud forms two quite distinct layers, one of which remains practically 

 undisturbed except in very rough weather, while the other is usually held suspended 

 in the water and only deposited in very sheltered places or at times of unusual calm. 

 This floating layer is of course very finely divided and habitually stains the water a 

 dirty clay-colour. Its occasional deposition is an unfavourable factor in the life of 

 many sessile organisms. The permanent layer is gray and of a clayey consistency, 

 but not so tenacious or so heavy as that of creeks and canals in the Gangetic delta. 

 It is mixed with a considerable amount of decayed vegetable matter, which some- 

 times stains it black, and often with a large number of small dead shells of genera 

 such as dementia, Theora, Nassa, Stenothyra , etc. These apparently do not remain 

 long intact; but at certain points, notably in the neighbourhood of Gopkuda Id., 

 there are fairly large deposits of dead Placuna-shells, which are evidently more 

 permanent, while at the edge of Rambha Bay masses of crude lime are dug from the 

 mud when the water sinks and with them occur large numbers of dead shells of Area 

 and Meretrix. These deposits of calcareous matter do not, however, seem to have 

 any direct effect on the fauna found amongst them. 



In the inner part of the outer channel there is a great mixture of mud and sand, 

 some of the latter being black and extremely heavy. Mr. G. H. Tipper of the 

 Geological Survey of India informs us that this is due to the presence of monazite in 

 small quantities. 



In the part of the outer channel that runs parallel to the Bay of Bengal, the 

 bottom is composed of almost pure yellow sand similar to that which forms the 

 beach along the greater part of the eastern shore of Peninsular India. The only 

 natural solid bodies found in this part of the lake are the large masses of dead and 

 living oyster-shells that lie in beds round the small islands opposite the village of 

 Manikpatna. The faunistic importance of the absence of solid bodies is illustrated 

 by the fact that on a small post set up to mark the channel near Satpara we found 

 several species not obtained anywhere else in the lake. 



