Sect. III. REEF-BUILDING CORALS L1YE. T09 



Besides these lithophytes, some fragments of the 

 Millepora alcicomis which occurs in the same relative 

 position at Keeling Island, were brought up ; and in 

 the deeper parts there were large beds of a Seriato- 

 pora, different from S. subulata, but closely allied to 

 it. On the beach within the reef, the rolled fragments 

 consisted chiefly of the corals just mentioned, and of 

 a massive Porites like that at Keeling atoll, of a 

 Meandrina, Pocillopora verrucosa, and of numerous 

 fragments of Nullipora. From 15 to 20 fathoms the 

 bottom was, with few exceptions, either formed of 

 sand, or thickly covered with Seriatopora : this delicate 

 coral seems to form at these depths extensive beds 

 unmingled with any other kind. At 20 fathoms, one 

 sounding brought up a fragment of Madrepora, ap- 

 parently M. pocillifera, and I believe it to be the 

 same species as that which mainly forms the upper 

 margin of the reef; if so, it grows in depths varying 

 from to 20 fathoms. Between 20 and 23 fathoms I 

 obtained several soundings, and they all showed a sandy 

 bottom, with one exception at 30 fathoms, when the 

 arming came up scooped out as if by the margin of a 

 large Caryophyllia. Beyond 33 fathoms I sounded 

 only once ; and from 86 fathoms, at the distance of one 

 mile and a third from the edge of the reef, the arming 

 brought up calcareous sand with a pebble of volcanic 

 rock. The circumstance of the arming having in- 

 variably come up quite clean, when sounding within a 

 certain number of fathoms off the reefs of Mauritius 

 and Keeling atoll (8 fathoms in the former case, 



