1 879.] Brazilian Corah and Coral Reefs. 543 



sandy and muddy bottoms, as from the rocky surfaces which 

 afford them a more secure foundation. Two or three additional 

 forms have been found along the coast ; but the soft nature of 

 these corals does not permit of their entering into the structure of 

 a reef, excepting as their minute spicules may be added to the cal- 

 careous sand or mud, a very important element in the formation 

 of coral reefs. 



We pass now from the corals proper to other organisms, that 

 give rise to quite as durable a substance of the same chemical 

 composition. These are among animals the Serpulae and Barna- 

 cles, and among plants the Nullipores. They grow abundantly on 

 the shore, forming encrusting layers or thickened masses ; but we 

 have treated very fully of these forms on page 352 of this journal, 

 in connection with the sandstone reefs. The thin encrusting 

 nullipores, with nearly smooth or slightly mammillate surface, 

 which cover so much of the Pernambuco reef, inhabit also many 

 of the rocky shores, but are more abundant over several of the 

 coral reefs that have reached too high a level for the growth of 

 true corals. There is another common nullipore, composed of 

 many intermingling and closely placed branches, which project 

 outwards in all directions as numerous digitations. This form 

 grows to an immense size at times, and is associated with the 

 corals in reef-building. A coarsely jointed coralline forms large 

 and dense clusters on the coast of Pernambuco, and its detached 

 segments sometimes form quite a thick deposit, especially on the 

 surfaces of reefs. 



The limestone-producing fauna and flora we have thus hastily 

 reviewed, may not differ in their essential features from those of 

 all the other coral regions in the world, but it has seemed best to 

 treat of them as fully as we have, in order to show more clearly 

 how few are the species of Brazilian corals, particularly those that 

 can aid materially in the building up of reefs. Up to this time 

 but thirteen species of Madreporian corals, representing ten 

 genera, have been found on or about the Brazilian reefs, and of 

 these only three or four ever attain to any considerable size. 

 Millepores have also contributed largely to the coral reefs. 



We have now to trace the gradual variation in coral growth as 

 we proceed outward from the shore toward the reef grounds. 

 Here is a rocky ledge, reaching to within about a fathom of the 

 surface at low tide. It is of small size, only a dozen feet each 



