i6o Memoirs of the Indian Museum. [Voi,. V, 



where the specific gravity of the water does not exceed i - oi5o. Its presence is par- 

 ticularly noticeable at places where rocks rise out of comparatively deep water and 

 masses of dead weed find lodgment and probably afford it food. Decaying algae seem 

 to be attractive to it, and when it is in their vicinity its stomach and alimentary canal 

 are filled with an opaque white substance that renders it relatively conspicuous. 



Macropsis orientalis swims in large shoals a short distance below the surface. 

 Each shoal, at any rate in the neighbourhood of rocks, "has its own " beat " to which 

 the majority of its members confine their movements. As a rule each individual 

 swims for the whole length of the ' f beat ' ' and turns when it comes to the end of 

 it, but sometimes single members of the swarm turn halfway and there seems to be 

 a tendency for all to move in an elongated figure-of-eight. The "beat" is never 

 more than a foot wide and may be from 3 to 6 ft. long. Its limits are determined 

 to some extent by the limits of the shadows cast by the rocks, for the animals 

 evidently avoid strong light. A few adventurers occasionally break from the shoal 

 and swim out sideways from it, but they always return to their company after a 

 short trip. Similar movements were noticed in specimens captive in an aquarium. 



Near the rocks at Ganta Sila at the south end of the lake a small cetacean, 

 Orcealla brevirostris , was noticed swimming backwards and forwards among shoals of 

 M. orientalis with its mouth open and apparently feeding upon them. Unfortu- 

 nately opportunity for a post mortem examination of the animal was lacking. 



Uriya fishermen of the lake catch large numbers of this Mysid by straining 

 water through a cloth. They mix them with turmeric, boil and dry the mass, and 

 eat it with rice. They say it is " very sweet" and the dish is known by the name 

 of netha ; the animals are called sridhar. 



Genus POTAMOMYSIS, Czerniavsky. 

 Potamomysis assimilis, Tattersall. 



P. assimilis, Tattersall, 1908, 1914. 



Further records : — Chittagong, pond at N.E. end of town, near the river, January 

 1913, coll. N. Annandale and S. W. Kemp. Common. 

 Chilka Lake, abundant everywhere. 



The telson of this species is subject to a considerable amount of variation. In 

 the Chilka Lake, the apex of the telson tends to be much narrower than in the types 

 and to have fewer spines, in some specimens as few as seven, which are larger than 

 in the type specimens and not arranged in series at all . The spines on the lateral 

 margins may be as many as thirteen. This lange in variation naturally gives the 

 shape of the telson a vastly different appearance in separate individuals, but all stages 

 of intermediates may be found. The amount of variation in the telson may possibly 

 be correlated with the enormous range in density of the water in the lake at differ- 

 ent times. 



Neomysis vulgaris in Britain is subject to variations in the arrangement of the 

 armature of the telson, which Norman suggests is influenced by the quantity of 



