264 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



and a half to two fathoms of water, beating upon the ledges much as 

 a small waterfall drops upon the rocks at its base. On the north shore 

 the breakers and sea do not act with the same regularity as on the 

 south shore. There, owing to the existence of an outer line of reefs, 

 the conditions are more uniform than on the north shore, where the 

 effect of the winds upon a comparatively broad stretch of sea are far 

 more variable. 



On the north shore the surface of the ledges between high and low 

 water mark is protected mainly by a small species of barnacle, clusters 

 of small Mytilus and incrusting Nullipores, and a few species of small 

 Alga?. At or below low-water mark Sargassum begins. Serpula? are 

 not as conunon as on the ledges off the south shore. On the north shore 

 serpuline atolls are most numerous on the ledges off Spanish Point in 

 an easterly direction for a distance of four or five miles. 



The mode of formation of the peculiar and intricate windings of the 

 vertical walls which crop up to the surface on the summits of the ledges, 

 and which take on such complicated curves can readily be explained 

 from the manner in which the ledges themselves yield to the action of 

 the surf from the wash of the sea, and also from the angle at which the 

 seolian strata lie when attacked by tlie waves. 



The following diagrams will further explain the mode of formation of 



the various serpuline reef struc- 

 tures whicli have been described. 

 AB (Fig. 1) is a piece of shore 

 cliff' which has become isolated 

 from the sliore ; tlie seolian lam- 

 ination is clearly seen above 

 high-water mark. Below high- 

 water mark it is honeycombed 

 and eaten away, leaving the 

 rcnlian pinnacle supported only 

 by a slender stem i-ising from 

 an extensive base more or less 

 covered with Alga'. Serpuhc, 

 and other gmwtlis. The surface 

 of the ledge, as well as tlie base 

 of the mass extending below 

 low-water mark, is more or less eaten away, and when the feolian pin- 

 nacle (A) has fallen off a mushroom-shaped mass is left, the upper sur- 

 face of which mav be above or below low-watcr mark. All trace of 







/ 



Fig. 1. 



