36 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



and^a half kilometres for the entire reef visible above water. North of 

 the bar the surf breaks upon the concealed continuation of the reef in 

 that direction. These breakers lie in the axis of the main I'eef, 



The various breaks and topographic relations of the reef are shown on 

 the accompanying map made by my assistant, Mr. C. E. Oilman, and 

 Deed not be verbally described. It will be seen that this reef is nearly 

 straight. In width it varies from twenty to seventy-five metres. 



On that part of the reef that stands out from the land across the 

 mouth of the river, and about six hundred metres south of the bar, is 

 an old fortress now surmounted by a lighthouse. 



Seen from a distance the surface appears almost perfectly flat. It 

 slopes gently toward the sea, but the bedding of the reef rock has a 

 somewhat steeper though still a gentle slope. In detail the surface is 

 in some places flat, in others it is etched in a manner characteristic of 

 all the sandstone reefs, and well illustrated in the photograph of the 

 etched surface of the Mamanguape reef. This etching is caused by the 

 removal of certain portions of the upper beds of the rock, and the leav- 

 ing behind of other and more resisting parts which stand out upon the 

 surface as sharp points or irregular slabs supported by short columns. 

 These jagged points are usually from a few centimetres to three decime- 

 tres high, but sometimes they are a metre high and make it difficult to 

 walk over the surface. 



Where the reef is broken and the surface has fallen in so as to be 

 lower than the general level, it is uncovered but little as compared with 

 the higher parts, and here the surface of the fallen blocks is overgrown 

 with barnacles and is black with seaweeds and corallines and is some- 

 times bored by sea-urchins. 



Along the outer or seaward face the reef is moi'e or less protected 

 from the force of .the waves by enormous slabs that have been dropped 

 where tliey now lie by the undermining of the original I'eef by the sea. 

 These slabs lie tipped about at various angles, but generally with their 

 outer ends dipping abruptly beneath the sea, and thus forming an 

 effective breakwater against the onslaught of the surf. Here and there 

 the outer face is broken off abruptly. The entire seaward face of the 

 reef is covered with corallines and other Algae, while the somewhat 

 protected parts are furrowed and bored by sea-urchins. The cavities 

 over this outer face of the reef and the seaweeds that grow there abound 

 in the forms of marine life that generally inhabit such places. 



The inner or landward face of the reef along its southern end lies 

 against the land, or rather the sands of the shore come down upon and 



