92 Couthouy on Coral Formations 



themselves in thin lamellse ; but these gradually disappear as 

 we recede from the surf, and finally abandon the field almost 

 entirely to the Madrepores. Ill calculated, however, by rea- 

 son of their fragility, to withstand the force of the breakers 

 that occasionally roll over the whole ledge, even these latter 

 are oaly found in small detached clusters, principally in the 

 little sandy pools and cavities which are scattered over the 

 surface, their greatest luxuriance being displayed in the clear, 

 tranquil waters of the lagoon, where they form submarine 

 thickets of great beauty, many yards in extent. 



The major portion of the plateau is encrusted by Niiliiporas 

 and a laminar deposition of carbonate of lime. In this are 

 imbedded multitudes of Tridacna3, the edges of whose man- 

 tles, as shown by the gaping of the shell, are so gorgeously 

 colored, that a correct representation of them would certainly 

 be thought exaggerated by one who had not seen the originals. 

 The varieties in this respect are very numerous, but the most 

 common are a deep vivid ultramarine blue or green, with dark 

 orange, purple or golden ocellations and wavy lines inter- 

 spersed. The marginal papillae are similarly ornamented. 



The Tridacnas appear like the Pholades, Lithodomi, &c., to 

 secrete a peculiar acid, enabling them to perforate calcareous 

 rocks, since, differing altogether in this from the shells imbed- 

 ded in branching corals, the situation in which they were gen- 

 erally found, was not caused by the growth around them of 

 the polyparia, nor by the deposition of calcareous matter. 

 This was evident at once, from their being in some cases im- 

 bedded in small clusters or masses of coral, which were cut 

 through by them in such a maimer that the parietes of the 

 cavity exhibited sections of the polyparia, transverse, vertical 

 or oblique, just as the Tridacna chanced to have worn its hole. 

 I have seen the base of large Madrepores, cut in this way, at 

 the origin of the branches, so that the upper portion of a 

 branch was on one side of the shell and the lower on the 

 other. The cavity is usually worn quite smooth, and fits 

 closely to the shell, there being often but just space left at the 

 surface for the valves to open about half an inch. They bury 

 themselves, beaks downward, and the basal margins of the 



