248 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



of seaweeds. Over the same zone, but not so apparent, spread encrusting 

 nuJlipores, which, though resembling lichens in form, are so highly charged 

 "with lime as to produce a hard coral-like substance. This is one of the most 

 important organisms living on the reef at present, and while aiding to protect 

 it from wear is also building it up. The barnacles and worm tubes of the 

 upper portion we have already referred to, and we have also statetl that over the 

 inner surface there seems to be nothing alive. As we enter the many open 

 pools and passage ways of the inner margin there is scarcelj' more to be seen. 

 Only here and there does a small mass of coral grow, usually a Siderastraea or 

 a Favia. Seaweeds and delicate tufted hydroids and bryozoans hang from the 

 eides of the pools, and a few shell-fish and star-fish lie on the sandy bottom. 

 Small, brilliantly-colored fish dart hither and thither, but the life is not what 

 we are taught to expect about a coral reef. 



" The features we have so far been giving are those of the northern section of 

 the reef. Going southward a short distance, the elevated outer mass gradually 

 diminishes in size, until it is reduced to a slightly raised border along the sea- 

 ward margin of a broad and flat reef. Still farther south the entire lower sur- 

 face, without the raised margin, seems lifted bodily upwards to form a high 

 massive wall, like that of an immense fort, flat above and perfectly square at 

 the sides. 



"Between the points of Penha and Cruz we find a varied structure, generally, 

 however, only a repetition of the forms already described. The reef is often 

 two or three times as broad as at Jaburu, but near its southern end it becomes 

 very irregular and much broken up, existing as a line of detached reef masses. 

 The passage ways through the reef are sometimes mere simple breaks, cut as 

 squarely and neatly as though the work of man ; at other times, however, the 

 edges of the reef bordering them are carried obliquely inwards some distance 

 toward the beach, enclosing a narrow entrance channel. These inner pro- 

 longations, although generally low and level, have the same structure as the 

 main reef. 



"Within the reef the water is always shallow; frequently the bottom lies so 

 high as to be quite exposed at low^ tide, and it is covered nearly everywhere by 

 a thick deposit of coral fragments, cemented together by carbonate of lime. The 

 corals are not in place but lie heaped together in every conceivable way, as 

 though they had been violently broken from the reef at some former time and 

 thrown inside by the waves. All the commoner forms are there, Millepora, 

 Siderastraea, Orbicella, and Mussa being the most conspicuous, and they are some- 

 times nearly perfect, but most often broken into irregular masses, large and 

 small. The majority are also coated over with a thin nullipore crust, as though 

 they had been dead a long time before they were swept from their jiroper dwel- 

 ling place. This coral deposit has considerable thickness near the middle of 

 the channel and thins out gradually toward the beach. 



" The extreme southern end of the reef is very low, and near to the beach. It 

 breaks down abruptly on the outer side, but on the inner is bordered by a thick, 

 consolidated layer, which reaches so nearly its own level that it is often difficult 



