104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



or small heads, Nullipores, Gorgonians, and debris of Mollusks torn off 

 from the outside of the reef by the incessant swell of the trade winds. 

 Owing to the steepness of the weather shelf there are not more than two 

 or three lines of breakers usually pounding on the reef. The corals 

 growing to seaward are almost entirely made up of large masses of As- 

 trseans and of a few heads of Madrepora palmata. There was a greater 

 variety of species on the lee side of the shelf, — Madrepora cervicornis, 

 small heads of Maeandrinas, Mauicina, and clusters of Millepora, as well 

 as large patches of Gorgonians. Nullipores are most abundant on the 

 summit of the reef, growing upon the smaller fragments of broken corals, 

 which they also often cement together, when they are forced inward into 

 deeper parts of the lagoon, where the cemented masses frequently form 

 heads of considerable size. 



Longitudinal and cross sections of the lagoon show that its bottom 

 is uniformly covered with coarse sand and broken shell material, or fine 

 sand, according to the distance from the action of the breakers. Upon 

 this looser material algae and corallines thrive and grow abundantly, 

 generally in large patches. As will be seen from the survey of the atoll 

 (Plate II. Fig. 1) the annular ring of corals within the 3 fathom line is 

 of nearly uniform width, except that on the northern edge the belt is 

 slightly narrower. On the shelf of the atoll, both seaward and on the 

 lagoon side, algse, corallines, and Nullipores are most abundant. On the 

 sandy bottom of the lagoon the patches of vegetation consisted mainly 

 of masses of Thalassia, Caulerpa, Penicillus, Halimeda, and Udotea, while 

 on the lagoon coral shelf are found masses of a species of Sargassum, of 

 Padina, Blodgettia, Laurencia, Digenea, and several species of incrusting 

 Nullipores. 1 The heads exposed at low tide are also more abundant on 

 the eastern face of the atoll, where there are no boat passages, while 

 on the south face, as well as on the northwestern face near Northwest 

 Cay, there are a number of points where the reef can safely be crossed. 

 There and on the spit to the west of South Cay the reef does not come 

 as near the surface, that end of the reef being lower both on the north 

 and south side of the atoll. 



The entrance to the atoll is on the west side. It is flanked on the 

 north side by Northwest Cay, and by the smaller South Cay about two 



1 For a list of the West Indian marine algae, the greater number of which have 

 been collected also at the Bahamas, see G. Dickie, " On the Marine Algae of Bar- 

 badoes," Journ. Linn. Soc, Botany, Vol. XIV. p. 146; and "Contributions to the 

 Botany of H. M. S. Challenger," communicated by Sir J. D. Hooker, "I. On the 

 Marine Algae of St. Thomas and the Bermudas," Ibid., p. 311. 



