256 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



walls of broader ledges which stand up from the bottom and seem to 

 have been built up by the organisms covering the surface. 



The serpuline atolls are of all shapes, depending primaiily upon that 

 of the slab from which they happen to be formed. We may imagine 

 one of the shore slabs or ledges more or less overgrown with Algse 

 and Serpulse exposed to the action of the incessant breakers of the 

 south shore. The sea face of the ledge either slopes rapidly or is more 

 or less vertical, sometimes undercut or worn to a mushroom-shaped 

 table. According to the hardness of the protected edge of the ledge or 

 of its surface, it becomes more or less broken through by incipient pot- 

 holes, which expose the softer seolian rock to the action of the sea. 

 With each tide the wearing action increases, until a circular pool is 

 formed, in wdiich sand is constantly triturating and grinding away the 

 softer surfaces. Thus a miniature inner lagoon becomes excavated, not 

 more than a few feet in depth, and surrounded by a more or less regular 

 rim ; the depth of some of the shallower lagoons varies from twelve to 

 fifteen inches. On a ledge in which a pot-hole has been formed the sea 

 thus washes at first into a shallow dish, or into a series of dishes which 

 are soon run together, and thus a straight or curved or S-shaped verti- 

 cal wall may be excavated on the edge of a ledge of seolian rock, the 

 inside depth in one case being eight feet. 



The serpuline atolls take their greatest development towards the 

 western part of the south shore. Off Great Turtle Bay we find the 

 same extraordinary development of the serpuline atolls and reefs which 

 we traced farther to the eastward, off Hungry Bay and off Elbow Bay. 

 There is hardly a sunken ledge on or along or off the south shore of 

 which the surface is not protected in some way by Algse and Serpula?, 

 and covered with structures which are directly the result of the action 

 of the sea upon the friable aeolian rock of which the ledges are com- 

 posed. It is indeed a remarkable sight to see, as far as the eye can 

 reach in either direction, this narrow belt of ledges which have been so 

 strangely modified by the action of the sea and the protecting agency 

 of the animal and vegetable growth upon its surface. 



The presence on the south shore of so many striking circular atolls 

 and horseshoe-shaped, crescent, or curved rings, or partial rings, and 

 S-shaped walls, withdraws the attention from the far greater number 

 of mushroom-shaped blocks and ledges which no longer reach the sur- 

 face, owing to the wearing of the reolian rock of which they are com- 

 posed. The atoll-shaped ledges have attracted more notice, not on 

 account of their greater number, but mainly from the interest centring 



