IV] LIFE-HISTORY AND ENVIRONMENT 47 



to suitable conditions of depth, temperature and 

 water, they must have some objects to which they 

 can firmly attach themselves. One does not find 

 pearl oysters, therefore, on the fine ooze which exists 

 further out from the island at greater depths. Again, 

 pearl oysters are not found on the hard bottoms from 

 twenty fathoms down to a hundred fathoms, although 

 in many of these places large encrusting organisms 

 like corals and sponges are found in great abundance. 

 The pearl-bank plateau off Ceylon is broken up into 

 a considerable number of paars, from three to eighteen 

 miles from land. The plateau is for the most part 

 composed of sand, but here and there the actual 

 bare rock appears, although in many places it is only 

 covered by a few inches of shifting sand. This sand 

 was examined by the author in connection with Pro- 

 fessor Herdman's visit to the pearl banks. It con- 

 sists of, sometimes, 40 — 50 7o of skeletons of certain 

 protozoa called Foraminifera, many of them attaining 

 the size of a threepenny-piece. The remainder is 

 made up of sand grains and calcareous animal remains. 

 The rock itself is only a "cemented sand," for the 

 sand grains in this sea-water with the calcareous 

 animal and plant remains become cemented together. 

 Lumps of coral and encrusting animal skeletons play 

 their part, and we have in consequence a modern 

 rock formation before our eyes. 



Many of the rocky blocks — composite structures 



