242 CEYLON MARINE BIOLOGICAL REPORTS. 



spat tliercforo from the Periya Paar is still veiy enigmatical, and the only conclusion one can arrive at 

 iu the present state of our knowledge is tliat in some waj' or other it is incident on the proximity of this 

 paar to the overfalls. It is further to be noted (i.) a large portion of this paar is sand, wliere oysters 

 could not possibly live ; (ii.) owing to the immense deposits, largo numbers, even if occurring on rock, 

 die from overcrowding ; and (iii.) that the bank is, par excellence, the home of large oyster-eating rays 

 and teleosts, and is recognized as the best fishing ground on the entire plateau. 



Finally, it is worthy of note that only one fishery has taken place on this paar (1879). The bank 

 lias been stocked w^th immense numbers of spat at least fifteen times during the last century, and in 

 oveiy case they have disappeared when about three to six months old. The disappearance has always taken 

 place during the north-east monsoon, and never during the south-west, and that in the only case where the 

 spat survived the first six months and thus acquired a firm foothold they lived to be adult and provided 

 a good fisheiy. This latter point is of very considerable importance, for it shows to us that if young 

 oysters occurring thereon are swept away by bottom currents, once they obtain a foothold they are 

 safe. This fact lends an interesting side light on the impossibility of older oysters (one to five years) 

 being swept away by currents on more protected areas. 



Li now deahng with bottom currents on the plateau in general, it is to be noted that our information 

 has been derived principally from three sources, and, as previoijsly noted, they refer only to the north- 

 east monsoon and to the area under lease. 



(1) Rough measurements of the bottom currents were made during two seasons by means of the 

 following apparatus : — A tow net was sunk by means of a small iron ring round the mouth to within 

 3 feet from the bottom. The depth over which the observations were made varied from day to day 

 between 5 and 10 fathoms. Assuming the depth to be 6 J fathoms, the tow net was suspended by 6 

 fathoms of rope to a flat bamboo float at the surface. This was placed just astern of the ship and allowed 

 to drift for one hour. The direction of the drift was noted, and the distance at the end of the hour 

 measured. This was done three times per day during two north-east monsoons. The average rate of 

 the current over the entire period was approximately 0-15 knot per hour. This method is of course 

 far from accurate, for it was found that the surface float was acted upon by the surface wind and 

 drift, and thus tended to drag the bottom net, which was but lightly sunk. The results were further 

 comphcated by the fact that during parts of March and April dead calms prevailed. Only on one occasion 

 was it found that the bottom current was running in an opposite direction to the surface current. The 

 above results are of too crude a nature to count for anything, save that if a strong bottom current had 

 existed the indication would certainly have been noted. 



(2) During the last six seasons (1906-1911) native divers have been continually at work on the 

 banks. It is exceedingly difficult to obtain precise information from divers, but only on very rare 

 occasions they have stated that there was a " little current " at the bottom. 



(3) The whole of the data on which we rely for our knowledge of the bottom currents has been 

 obtained by one of us by means of the diving dress, and this work has been conducted over two seasons. 



Although, as noted, dress diving has only been carried on during the north-east monsoon, the 

 bulk of the work was done very early, before the south-west monsoon had fully subsided, and when 

 one would naturally expect to find the accumulated and undisturbed results of the monsoon on the 

 floor of the ocean. It is on this account that we place some reUance on our data, for although our 

 observations were not made during the actual south-west monsoon, stiU, if a bottom current existed 

 for several months, there would certainly be some indication of it at the bottom, particularly over the 

 sandy areas which here and there alternate with rock. 



This sand is divisible into two kinds : a loose, coarse-grained, angular kind, usually occurring on 

 rock, and mixed with sliell debris, and a fine-grained kind. Usually they do not co-exist at the same 

 place, but if thej' do, one kind is predominant. The fine-grained sand is more or less consoUdated, and is 

 hard, and caked, and not yielding when trod upon. The coarse-grained angular sand may be considered 

 as water-logged, for o\ving to its angular nature there are considerable interspaces between the grains 

 filled with water, M-liich do not exist in tlie fine-grained variety. Consequently it is found, and has been 

 proved over and over again by means of the diving dress, that the coarse-grained sand is more susceptible 

 to bottom movements than the fine-grained sand. 



