144 CEYLON PEARL OYSTER REPORT. 



of the opening, which is a regular oval, while that of Borradaile's (9) P. madrepores 

 from the Maldives, the shell of which is very similar, is rhomboidal. 



Weltner (5) has already recorded this species from Ceylon (on Prionastrcea 

 acuticollis). It was originally described from the Red Sea, and is abundant on 

 specimens of Turbinaria and Porites collected by the late Dr. J. Anderson in 

 Mergui. 



Professor Hurdman's specimens occur on both the upper and the lower surface of 

 the Turbinaria, but chiefly on the latter. In either case they have been covered by 

 the living tissues of the organism while still small. In younger individuals the basis 

 is almost flat as a whole, but deeply sulcated and with an oval depression, corresponding 

 to the opening of the shell above, in the centre. The shell is conical. As the 

 calcareous substance of the coral is deposited round them, the shell becomes relatively 

 natter and the base deeply concave from within. The whole animal is finally buried. 

 An opening is retained for a time by the action of the cirri and possibly of the 

 opercular plates ; but in some examples examined even this has been closed over and 

 the Barnacle has perished, its presence being indicated by a smooth, rounded mound 

 on the surface of the coral. The division between shell and basis can usually be 

 detected, however, on the surface of the coral. A cyst in which the opening still 

 remains open measures 9 millims. in depth, 14 millims. in length, and 10 millims. in 

 breadth, all the measurements being taken externally. The shell is about 1 millim. 

 thick above, but slightly thicker at the point where the valves meet the basis. The 

 internal depth is a little over 8 millims., of which the basal portion occupies 5 millims. 

 The basis is barely distinguishable from the substance of the coral. In some Mergui 

 specimens the cyst is twice as deep and far more protuberant on the surface of the 

 coral, but the opening of the shell is still patent. 



*Tetraclita serrata, Darwin. 



Locality : On a dead Heteropsammia off Ceylon. 



The presence of the operculum enables me to identify the single specimen obtained 

 by Professor Herdman, which is small (antero-posterior diameter of base = 5 millims.). 

 The species does not appear to have been recorded hitherto from the eastern part 

 of the Indian Ocean. 



# Acasta cyathus, Darwin (2). 



Locality : South end of Cheval Paar, Gulf of Manaar. 



The specimen is longer and less spheroidal than the one figured by Darwin, but 

 otherwise agrees closely with his figure. The shell is white instead of pink ; but this 

 may be due to the action of spirit. The basis, while nearly flat as a whole, bears 

 a circular depression in its centre, if the shell be viewed from below. This is probably 

 due to the presence of a minute Polycheete, which is coiled up, inside a transparent 

 tube, within the depression. 



