240 Records of the Indian Museum. [Vol. XIII, 



Notwithstanding these facts I am strongly of the opinion that 

 the resemblances between the two faunas are not fortuitous ; some 

 underlying cause must be at work and there can be no reasonable 

 doubt that this cause is to be sought in the environment. So far 

 as I can understand, the environment appears to have exercised a 

 selective influence on the Matlah fauna, and has in some way per- 

 mitted the existence only of those species that conform to certain 

 definite rules. That this has resulted in the existence of a fauna 

 resembling that of the deep sea is exceedingly curious, but it 

 affords, I think, a clue as to the factors involved. 



There are few points of similarity between the environment 

 of the species found in the Matlah River and that of those found 

 in the ocean depths. Complete absence of light, great pressure, 

 low temperatures, high salinities and still water characterise the 

 latter, whereas in the former the temperatures are high, the salini- 

 ties very low and the tidal currents swift. I have no precise 

 information as to the amount of light on the bed of the Matlah 

 River. It is no doubt greatly diminished, for the water is 

 heavily laden with silt and, as has already been pointed out, the 

 upper layers of mud are probably always kept in motion by the 

 tidal currents. There can, however, be little doubt that some 

 light penetrates to the bottom. 



But there is another factor, which may or may not be depend- 

 ent on the amount of light, that appears to be of considerable 

 importance; to this factor the term visibility ma3' be applied. 

 Dr. Annandale and I noticed that the Palaemonidae found in the 

 Matlah River, when placed in an aquarium with the cleanest 

 river water we could procure, were quite invisible unless they 

 approached within an inch or so of the glass. The lack of visibil- 

 ity was brought about in the main by the colour of the animals, 

 the milky translucency of their bodies seeming to correspond pre- 

 cisely with the turbidity of the water. Transparency is quite in- 

 effectual in rendering animals invisible in muddy water ; I have 

 frequently noticed that such planctonic forms as Pleurobrachia 

 and the Penaeid xicetes are extremely conspicuous in silt-laden 

 water, forming as it were hyaline spaces in an otherwise merely 

 translucent medium. In aquatic forms, then, the factor of visi- 

 bility seems to depend, when light is present, on a relation be- 

 tween the opacity of the animal compared with that of the water 

 in which it lives. In the Matlah River visibility, in the case of a 

 considerable part of the bottom fauna, is evidently very low and 

 in the deep sea, unless animal or bacterial luminosity is strong, it 

 is practically absent. 



Thus in the matter of visibility there is perhaps some slight 

 similarity between the two environments and other factors com- 

 mon to both are the absence of vegetation and the nature of the 

 bottom. But weeds are in many places absent without producing 

 the curious effects seen in the Matlah fauna, and it is probable 

 that the character of the bottom is much the more important. The 

 mud of the Matlah River bed is of a peculiarly soft consistency 



