ANATOMY OF THE PEART, OYSTER, 41 



The markings on the interior of the shell (Plate IV.) consist of: 



(1) The large adductor impression sub-centrally placed and occupying from one- 

 third to one-half the diameter of the shell : and 



(2) The pallial line and scars caused by the insertion of the pallia! muscles, which 

 are fan-shaped bundles formed of fibres radiating outwards from small insertions 

 placed along the pallial line. These insertion scars vary considerably both in number 

 and in form. Usually there are from 12 to 15 between the umbonal region, where 

 they begin anteriorly, and the antero-ventral end of the adductor, with 3 between 

 the dorsal tip of the latter and the hinge line (see Plate IV., figs. 2 to 6). Besides 

 these, which are distinct scars, there is an extremely narrow and practically con- 

 tinuous insertion band confluent with the posterior and ventral edge of the adductor 

 scar. This band leaves no separate impress upon the nacreous surface of the valve, 

 its scar, like that of the retractor muscle of the same side, being merged with that 

 of the adductor (see Plate IV., fig. 1). Figs. 2 to 6 show five variations in the 

 distribution of the insertion scars of the pallial muscles, while fig. 1 is the typical 

 arrangement of the muscles as seen on removal of the valve. 



As to the size of the Ceylon pearl oyster shell at different ages, we believe from 

 our observations on different grounds that there may be very considerable variations 

 according as the conditions are favourable or the reverse. It is difficult to get well- 

 established dates fixing the ages, but the following figures may be useful as giving 

 some indication.* 



A pearl oyster in the Master Attendant's Office at Colombo, labelled by 

 Captain Donnan as being 2^ years old, measures 3 X 2f inches. 



The average of the oldest pearl oyster from the Muttuvaratu Paar in March, 1902, 

 also considered by Captain Donnan to be 2^ years old, is 2\ X 2 inches. 



A 1 -year-old oyster, from the samples in the Master Attendant's Office, measures 

 2\ X 2 inches. 



Some natural-size drawings made by Dr. Ondaatje many years ago, at Colombo, 

 show the following sizes : 



1 -year-old measures 2X2 inches. 



2 ,, ,, ,, 3 X 2j ,, 



3 ,, ,, ,, 3 2 X 3 ,, 



Mr. Hornell is now measuring very large numbers of shells, and is determining 

 more accurately than has yet been done the average growth year by year, and the 

 results of his observations will appear in the final volume. 



* See also the measurements and weights given in " Observations and Experiments on the Life-History 

 and Habits of the Pearl Oyster," in Part I. of this Report, p. 136. 



G 



