140 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
barrier reef stretching along the south coast of Viti Levu, especially at 
Lauthala Bay, Suva (Plate 7), and the reef harbors and passages between 
Suva and Serua (Plate 5), and out of the smaller atolls like Motua lai lai, 
Thakau Leka leka, Thakau Momo, and others (Plates 12, 18, 21). 
The strength, of the currents in the channel separating the barrier 
reef and the shore has been noticed by Semper? and by Mébius? as 
bearing an important part in Mauritius. There is in the Biologisches 
Centralblatt, 1889-90, Bd. IX. p. 564, a short review of the third edi- 
tion of Darwin’s “Coral Reefs,” showing the principal points in the 
discussion of the reefs to which Bonney has called attention. But this 
discussion is mainly theoretical, and adds no new factors in the problem. 
I would refer to what Gardiner *® says regarding the conditions affecting 
the growth of coral reefs in Fiji, where he shows the effect of tidal 
currents in the passages of reefs and inside of reefs. Strong currents 
prevent the coral larve from fixing themselves in localities which they 
scour, while the corals will thrive off the passages where the currents 
have lost their strength. The navigators and naturalists of the U. S. 
Exploring Expedition* frequently speak of the rapid outward current 
passing through the openings of the reefs, especially during the ebbing 
tide. Dana® had noticed the great strength of the tidal currents, and 
he well explains by their action the great diversity of distribution of 
material over the bottom of a lagoon or of a barrier reef channel. 
There seems to be no question that the action of the sea can cut out the 
lagoons of barrier reefs and of atolls at the depths at which they have been 
observed in the Fijis. Although there are individual atolls which show 
depths of thirty-five to fifty fathoms and even more, these are excep- 
tional depths, which are readily explained as due to other causes than 
the scouring action of the sea. 
Admiral Wharton ° has given an excellent summary of cases showing to 
what depths the action of the sea in motion may be felt to a sufficient ex- 
tent to move material at depths of fifty to sixty fathoms. As he justly 
says, “ The effect [of the action of the waves in an otherwise deep sea 
over which strong winds are continually blowing] will be to cut down an 
island more or less rapidly, according to its constitution, toa very con- 
siderable depth below the surface, the final result being a perfectly flat 
bank.” 
1 Natiirl. Existenzbeding. d. Thiere, Bd. II. (1880), chapters 7, 8. 
2 Beitrage z. Meeres Fauna der Insel Mauritius, Berlin, 1880, p. 29. 
8 Loc. cit., p. 484. * Dana, p. 170. 
5 Loc. cit., p. 151. ® Nature, Vol. LV. p. 392. 
