Sect. II. RATE OF GROWTH. 105 



shipwreck, unfortunately prevents its being known 

 to what' genera these corals belonged ; but from the 

 numbers experimented on, it is certain that all the 

 more conspicuous kinds must have been included. 

 Dr. Allan informs me, in a letter, that he believes 

 it was a Madrepora which grew most vigorously. 

 One may be permitted to suspect that the level of the 

 sea might possibly have been somewhat different at the 

 two stated periods ; nevertheless, it is quite evident 

 that the growth of the ten-pound masses, during the 

 six or seven months at the end of which they were 

 found to be immovably fixed l and several feet in 

 length, must have been very great. The fact of the 

 different kinds of coral, when placed in one clump, 

 having increased in extremely unequal ratios, is very 

 interesting, as it shows the manner in which a reef, 

 supporting many species of coral, would probably be 

 affected by a change in the external conditions 

 favouring one kind more than another. The growth 

 of the masses of coral in N. and S. lines parallel to 

 the prevailing currents, whether due to the drift- 

 ing of sediment or to the simple movement of the 

 water, is, also, an interesting circumstance. 



Lieut. Wellstead, I.N., informed me that in the 

 Persian Gulf a ship had her copper bottom en- 

 crusted in the course of twenty months with a layer 



1 It is stated by Mr. De la Beche (Geological Manual, p. 143), on 

 the authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that 

 some specimens of Polypifers, placed by him in a sheltered pool of 

 water, were found in the course of a few days firmly fixed by the secre- 

 tion of a stony matter, to the bottom. 



