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



depth, and the variations of level that occur at different seasons, though relatively 

 great (about 5 or 6 ft.), are not sufficient to have any appreciable direct effect 

 on the fauna. The bottom of the main area of the lake is covered with soft mud, 

 which probably overlies a deep layer of clean sea-sand, while along the outer shore 

 of this area and round some of the lower islands, sand and mud are mixed. At 

 most places at which this occurs the water becomes excessively shallow in the dry 

 season and is so heated by the sun that conditions are inimical to most forms of animal 

 life ; but the admixture is a marked feature of the bottom round the island of Nal- 

 bano and a belt of gritty mud extends in fairly deep water out into the channel by 

 means of which the lake is connected with the sea. The outer part of this channel, 

 near the sea-mouth, is scoured by currents at certain seasons and its bottom consists 

 of clean sand. At several points along the inner shore of the main area and on some 

 of the islands there are rocks, partially or entirely submerged in summer and autumn, 

 but in spring exposed and dry owing to the sinking of the water-level. In sheltered 

 spots, where the water near the shore is relatively deep, dense thickets of weed grow 

 up in autumn, dying out almost completely in the rainy season. They consist for the 

 most part of a species of Potamogeton that sometimes attains a height of at least five 

 feet in order to flower on the surface . A fine-branched alga also forms somewhat lower 

 thickets at a few places in both parts of the lake-system. 



For the greater part of the year the whole of the main area is filled with water 

 that may be called brackish, having a specific gravity (corrected) that reaches a 

 maximum of 1-0150. At this period there is an abrupt change in salinity at the 

 point where the outer channel opens into the main area, the water in the former 

 being, at the height of the season, as salt, or very nearly as salt as that of the Bay 

 of Bengal outside the sea-mouth (sp. gr. 1-02650). Between August and October 

 floods of fresh water pour in from the rivers at the north, driving before them the 

 salter water, until the northern part of the main area and the whole of the outer 

 channel become entirely fresh. In a comparatively small area at the southern end 

 of the lake into which no rivers open, the floods have less effect and the water 

 remains brackish throughout the year, the specific gravity varying from 1-003 t° 

 1-015. The northern boundary of this area is situated close to the island of Kalidai, 

 which forms a land-mark in the distribution of species. 



The specific gravities recorded in the table on pp. 332, 333 represent not the full 

 range of salinity in which the species may occur, but merely that in which we found 

 living specimens in the Chilka L,ake. 



In the collection of the Indian Museum there are specimens of some 37 named 

 species of Gastropods and 47 of L,amellibranchs from the lake. Certain shells that 

 we believe to have been introduced by man or other agents may be dismissed very 

 briefly. We have no reason to include these species among the living fauna, for the 

 Gastropods are merely represented by dead shells, most of which were occupied by 

 hermit-crabs, while of the Lamellibranchs only single valves were obtained, in cir- 

 cumstances which suggested that they had been brought from the coast by man. 

 These introduced shells are : — 



