146 CEYLON PEART, OYSTER REPORT. 



broad, longitudinally ridged, oblique. External surface of parietes and radii rough 

 and irregularly spiny, with spiny, cylindrical processes connecting tbe basal part 

 of the rostrum and the laterals with- the coral to which the Barnacle is attached. 

 Internal surface of parietes strongly sulcated longitudinally above and vertically 

 below. Shell fragile, mound-shaped, generally narrower in front than behind, basis 

 flat. Opening more or less vertical, irregularly oval, strongly denticulate. Colour, 

 dirty white ; where covered with epidermis, yellowish ; tinged, more or less deeply, 

 with steel-blue above. Scutum triangular, with muscular depressions feebly developed ; 

 tergum pointed above, much wider than high, with a broad, bluntly rounded spur. 



Several dried specimens on the lower surface of Turhinaria. 



T have been in much doubt as to the proper generic position of this form. As the 

 basis is absolutely flat it can hardly be put in Darwin's Section B of Balanus, with 

 which its true affinities may lie. On the other hand, its more or less globular 

 outline, its fragile shell, and the shape of its compartments certainly ally it to 

 Acasta, if this group can be given generic rank. The periphery of the basis is 

 pierced by small irregular holes as in some species of Acasta. It is almost impossible 

 to lay down an exact line between Acasta and Section B of Balanus; but in most 

 cases there is no difficulty in seeing a difference as regards species. 



No species of Acasta has hitherto been recorded from a Madreporarian coral. The 

 present form occurs with Balanus allium, B. tcrcbratus, and Pyrgoma conjugatum, 

 and it is interesting to note the different ways in which the four species have been 

 adapted for the same mode of life. With /'. conjugatum I have already dealt. 

 B. terebratus anchors itself among the living tissues of the coral by means of 

 prolongations of the outstanding vertical ridges on its shell. These tissues do not 

 grow over it as they do over the Pyrgoma, at least, in the specimens examined. By 

 deepening its base, it can compensate to some extent for the growth of the calcareous 

 part of the coral ; but it appears to be continually thrust outwards on to the surface, 

 and none of the specimens show any signs of being engulfed. B. allium, as far as one 

 can see, is more frequently destroyed, several specimens having been completely 

 buried. Possibly the peculiar smooth, glistening surface of their shell may be, 

 however, in some way a protection to them, although the smoothness is of texture 

 rather than actual surface. A. funiculoriuu is a more highly specialized form than 

 either of the preceding two. Its cables, so to speak, are not prolongations of such 

 ridges as occur on other species, but seem to be special structures, allied to the spines 

 which occur on the shell of A. cyathus, but very much more highly developed ; their 

 closest homologues being the processes on the shell of Balanus tintinnabulum, var. 

 spinosus. By means of them the Barnacle anchors itself to the slippery surface of the 

 coral. If the coral commences to grow round it, it can save itself for a period 

 by elongating the posterior part of its shell, so that the opening may approach the 

 horizontal more closely than it usually does; but individuals are frequently buried 

 and perish. It may be noted that while the Pyrgoma, which can be surrounded by 



