260 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The diminutive reefs formed off the south shore of Bermuda are of 

 all possible shapes, — atolls with regular rims of the same width on 

 all sides, atolls with the sea face rim wider than that to the leeward, 

 and parts of rims of circular atolls of horseshoe or crescent shape, or 

 only of.parts of arcs of greater or less extent. We also find belts of 

 small atolls on ledges of considerable width, and atolls of an elongate 

 type ; others are dumbbell-shaped, formed evidently by the breaking 

 through of division walls of circular atolls. We also meet with chains of 

 atolls, each one forming a link as it were, or irregular parallel chains, 

 which, when the separating walls are broken through, give the elements 

 for all the possible figures assumed by the ledges off the south shore. 

 When such reefs are formed on the shore ledges, we have all the possible 

 types of fringing and barrier reefs, or combinations of these, forming 

 diminutive l'eefs with low vertical walls apparently most irregularly 

 placed, often as if their existence in their varying shapes and positions 

 could be due only to the upward growth of the Serpulae and Algae. But 

 I believe that the vertical growth of Serpulae and Algoe is not of itself 

 sufficient to account for the existence of the vertical walls, and that they 

 are due only in small part to the upward growth of organic material, 

 and in a great measure to the action of the breakers upon the aeolian 

 rock ledges, probably in the manner I have just described. 



The barrier reef off the small spits to the east of Whale Bay at the 

 Targets may be described as a vertical wall surrounding three sides of a 

 rectangle, the diagonal of which is somewhat over fifty feet ; the sea face 

 corners are well rounded and the side walls formed of short arcs. The 

 distance from the shore edge of the ledge to the outer wall is about 

 twenty-two feet. The greatest width of the rim is at the two outer 

 angles, where it varies from five to six feet ; the inside edge of the sides 

 of the rim gradually passes into the shore ledge, being in a general 

 way parallel to the trend of the sides of the barrier reef. The raised 

 part of the rim varies from twelve to sixteen inches in height. The 

 outer vertical wall has a height on the west of eight feet, and on the 

 east of about six feet, the slope of the ledge being more or less con- 

 centric round the deepest part as a centre, and sloping sideways towards 

 the shore edge. The outer rim at two points is gouged out into two 

 smaller elongated pits. The rim is everywhere well protected by Alga?, 

 Corallines, and Serpulae, the Algse growing on the more or less level 

 platform of the rim, and on the outer and inner faces of the vertical 

 walls of the barrier reef. The ledge is greatly undercut, and its outer 

 faces present all the irregularities of wear by water so characteristic of 



