128 



MADRAS FISHERIES BULLETIN 



[VOL. XVI, 



after time it seems that wide deposits of the spat of this mollusc, 

 clustering on weeds and stones, have been mistaken by former 

 Superintendents of the Pearl Banks for the true pearl oyster. For 

 reference, I give an enlarged drawing of a young Avicula vexillum 

 at the age when it has been so mistaken (Fig. 8 B.) Figure 8 B 

 of the form of true pearl oyster spat is provided for comparison. 



Fig. 8. Comparison of the spat of the true pearl oyster (A), with that of Avicula 

 vexillum, the so-called il false-spat" (B). 



Examination shows differences sufficiently distinct to render des- 

 crimination fairly easy. The chief of these are in the colouration 

 and the outline. False spat is more brightly and profusely 

 coloured with reddish radiating bands, and these occur from an 

 earlier age. More distinctive still is the markedly oblique form 

 of the false-spat, and particularly the way the lower posterior 

 angle is prolonged backwards into a long rounded point. In pearl 

 oyster spat of the same age, the colouration is distinctly less 

 abundant and clearly marked, while the obliquity of the valves 

 is slight. As in the adult the tendency is towards a rounded sub- 

 quadrangular outline. These differences however do not show 

 themselves clearly till the spat attains a length of about two 

 millimetres; in the veliger and very early fixed stages there are 

 no distinguishable differences between the larvae of the two 

 species. 



IX— PEARL FORMATION IN THE INDIAN PEARL 



OYSTER. 



In the following brief note, I do not propose to traverse the past 

 history of the theories and researches upon this subject ; the con- 

 clusions to which I have come after twenty years' work on this 



