164 CRUISE OF THE BARRERA 



materials so exposed consist of these various coral 

 residues, and an exceedingly small portion of the 

 whole consists of the actual coral itself. Further- 

 more much of this coral detritus is obliterated as 

 such by the processes of atmospheric welding into 

 rock. Masses of coral heads, bored and riddled 

 by many living creatures, become filled with sand 

 and mud, shells, and the tests of many lowly or- 

 ganisms, and when finally exposed to the air and 

 rain-water the whole undergoes certain changes, 

 the various component parts being partially dis- 

 solved and cemented and recrystallized to a greater 

 or less extent. In such limestones the coral 

 structure can only be detected by a microscope. 



Throughout the Antilles the narrow little strip 

 of honeycomb limestone upon the shore edge is 

 peopled by a very similar group of animal forms. 

 One would naturally suppose that so inhospitable 

 a station would never be selected by any living 

 creature for its permanent home, yet the little 

 assemblage of species here found have become 

 so adapted to their uncomfortable surroundings 

 that they are practically never encountered 

 elsewhere. Several crabs are numerous, particu- 

 larly the red and black Goniopsis cruentata. These 



