20 THE BERMUDA ISLANDS. 



larger or more conspicuous shells of the archipelago are found 

 here, nor indeed, can it be said that shells of any description 

 are numerous. 



The predominant form of coral, at least in the upper layer, 

 is the Porites, whose masses of bright orange appear here and 

 there through the more sombre tints of the serpula by which 

 they are almost everywhere covered. It grows to within a few 

 inches of the water-line, but nowhere did we see it protrude 

 above the surface, even at lowest water. This is true of all the 

 stone-corals with which we came in contact, and also of the 

 millepore. But we found large encrusting patches of that sin- 

 gular actinioid form, Cortidfera flava, completely exposed, and 

 beyond the reach of spray. The length of exposure is, how- 

 ever, short, and probably sufficient water is retained during 

 this time within the cavernous mass to minister properly to the 

 wants of the organism. 



The serpula grows in dense bunches, forming a true base- 

 ment, which is admirably adapted toward withstanding the at- 

 tacks of the sea. Indeed, everywhere along the border where 

 the surf beats hardest, the serpula growth was most largely de- 

 veloped, and to such an extent as to form a raised rim or bar- 

 rier to the more protected inner side. Breaking in on all sides 

 the surf has created a number of more or less irregularly oval 

 islets with depressed centers — or, more properly, with elevated 

 borders — diminutive atolls, as it were ; and, indeed, this struct- 

 ure has led naturalists to assume that the form of the true 

 coral atolls, with their central lagoon and bounding outer ring, 

 may have been produced in much the same way, and without 

 the assistance of any such subsidence as was considered neces- 

 sary for their formation by the late Mr. Darwin. I feel satisfied, 

 however, that the two structures, while seemingly alike, have 

 practically little or notliing in common ; in the one case the 

 central depression is merely a negative one, being such by 

 reason of a somewhat more rapid growth developed only from 

 the water-line, or within the surf; while in the other, the 

 hollow extends frequently to depths far beyond the zone of 



