96 



MADRAS FISHERIES BULLETIN 



[VOL. XVI, 



Making use of the former method we find usually but little 

 increase in the length and depth of the shell after the third year, 

 the shell-secreting energy of the animals being thereafter occupied 

 chiefly in adding to the thickness of the valves. 



I now attach greater importance to observations upon the 

 average weight of oyster shells than upon measurements of length 

 and depth, increase being nearly as steadily progressive in old age 

 in the case of weight of shell as it is during the first three years of 

 existence; it furnishes us with the most reliable guide available in 

 the assessment of age that I know of. 



But we need to have considerable knowledge of the special 

 growth peculiarities of the ground we deal with. Some pars by 

 reason of abundant food supply hasten the growth of their oysters 

 to a surprising degree, while others where less favourable condi- 

 tions prevail bear oysters of an unhealthy appearance and of 

 stunted size. 



On the Ceylon side two distinct types of oysters are found, the 

 one large and vigorous, peculiar to the Southern and Eastern 

 Cheval and Moderagam Pars, the other slow-growing, small and 

 stunted, characteristic of the rocky banks of the Muttuvaratu, 

 Mid-West and North-West Cheval. 



We will now proceed to compare the sizes and weights of the 

 generation of oysters carefully guarded on the Tolayiram Par by 

 Captain Phipps from 1884 to 1889 with those of oysters of the two 

 types referred to on the Ceylon side. 



Weight of Pearl Oyster shells. 



