262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



ber of the possible intricate figures that are found on the aeoUan ledges 

 off the south shore. The Algae, Serpulce, and other organic growths 

 which thrive upon the edges of the ledges and spread down upon the 

 vertical faces, still further protect tlie sides of the mushroom tables 

 from being washed or eaten away by the forward and backward rush of 

 the sea. 



Off the same locality an elliptical atoll forty by thirty feet, standing in 

 a depth of ten feet at low water, formed the spreading top of a mushroom- 

 shaped ledge from the vertical sides of which all traces of the seolian char- 

 acter of the substructure had disappeared. Where not overgrown with 

 Algaj and Corallines, it showed the peculiar gouging out and honev 

 combing character of all shore rocks, either when exposed at low water 

 or where extending below it. The inner part of tlie weather rim was not 

 quite parallel with the outer outline ; it projected in the centre and was 

 somewhat scalloped in outline, with a few small deep pits. The rim 

 varied in width from five and a half feet on the weather side to one 

 foot in its narrowest part on the leeward edge. I could not detect that 

 the weather rim was perceptibly higher than the leeward rim, though 

 this is not unfrequently the case.^ The pot-hole was six feet in depth 

 nearer the western edge, gradually decreasing in depth to three feet 

 on the opposite side. A crescent-shaped lagoon eighteen feet in diame- 

 ter, greatly undercut, especially on the weather side, as all these 

 ledges are, had a rim five feet wide at its widest part, gradually taper- 

 ino- to four or five inches at the two extremities of the crescent. Both 

 this and the elliptical atoll just described had a narrow shelving plat- 

 form on the inside of the weather rim. The lee edge of this crescent- 

 shaped atoll was worn away five feet lower than the weather rim, A 

 circular atoll twenty-five feet in diameter was surrounded by an irregu- 

 larly elliptical rim twice as wide on the weather as on the lee side, form- 

 ing a scalloped pot-hole with a greatest denth of three feet. Along the 



1 Heilprin speaks of the Serpulae as occurring " in dense bundles," and where 

 the surf beats liardest " tlie Serpula growth was most largely developed, and to such 

 an extent as to form a raised rim or barrier to the more protected inner side." 

 But, as he himself says, (and I am not quite clear whether he attributes the for- 

 mation of the atoll to tlie Serpulae,) " the breaking in on all sides of the surf has 

 created a number of more or less irregularly oval islets with depressed centres, or, 

 more properly, with elevated borders." In the one case he says, " The 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," but he feels satisfied, how- 

 ever, that the two structures (these serpuline atolls and the coral atolls), while seem- 

 ingly alike, have practically little or nothing in common. 



