12 Memoirs of the Indian Museum. [Voe. V, 



in level is that the water of low salinity, hitherto confined at the southern end, 

 spreads further north. In course of time the level sinks to a minimum and subse- 

 quently, under suitable conditions of wind and tide, volumes of salt water enter 

 from the sea and entirely fill the outer channel. This in 1914 had already taken 

 place before the month of February. Under normal conditions the waters of the 

 main area probably rise in salinity, owing to successive inflows from the Bay of 

 Bengal, until a maximum is reached in July. By August the monsoon floods have 

 commenced, the water-level rises rapidly and a repetition of the annual cycle 

 begins. 



The important subject of salinities may therefore be summarized as follows: — 



(1) In the dry season the water of the outer channel is practically as salt as 



that of the Bay of Bengal, while that of the main area is distinctly 

 brackish. 



(2) At the end of the wet season the water of the whole of the outer channel 



and of a great part of the main area is fresh, while that of the south- 

 western part of the latter is but slightly saline. 



(3) At all times of year the change from water of low to that of comparatively 



high salinity take places abruptly in a very limited area, so that the 

 isoh alines are closely crowded together. 



(4) In the dry season this area of abrupt change is situated at the junction of 



the outer channel with the main area, but by the end of the wet season 

 it has shifted to the sea-mouth. 



Variations in the temperature of the water of the lake have probably, except 



in extreme cases, but little influence on the distribu- 

 tion of the fauna. According to our observations, the 

 surface temperature ranges from 25 to 35°C. and is probably higher to a marked 

 extent than that of the Bay of Bengal. The cooler water is naturally found in the 

 more central parts, while nearer the shores, and especially in the vicinity of rocky 

 headlands, the temperature is noticeably higher. Even comparatively short periods 

 of hot weather must obviously have a marked effect in raising the surface temperature 

 and the maximum must be reached in very shallow water or in small more or 

 less isolated pools at the margin. In one such spot we obtained, in March, a reading 

 of 43 °C. ; this temperature must be inimical to many forms of life and as a 

 matter of observation few living animals are to be found in situations of the kind. 

 Seasonal variation in temperature is certainly not very great : but our data are not 

 sufficiently extensive to permit of a more precise statement. 



Vegetation. 



In most parts of the lake the aquatic vegetation is scanty, but in a few sheltered 

 bays in the main area a species of Potamogeton 1 with slender, grass-like leaves grows 



1 Probably P. pectinatus, Linn. We have to thank Dr. D. Hooper for the name of this plant. 



