440 Mr. G. C. Bourne. The Atoll of Diego Garcia [Mar. 22, 



common ancestral type. These relationships may be thus indicated, 

 taking only a few of the characters of each. 



Lizard. 



Cerebellum small, optic 

 lobes meeting, paroccipi- 

 tal formed chiefly by the 

 opisthotic. 



~Pterosawrian, 



Cerebellum large and 

 between optic lobes, par- 

 occipital formed chiefly 

 by the opisthotic. 



Bird. 



Cerebellum large and 

 between optic lobes, par- 

 occipital formed chiefly 

 by exoccipital. 



Ancestral 



Cerebellum small, optic 

 lobes meeting, paroccipi- 

 tal small, and formed by 

 both exoccipital and opis- 

 thotic. 



II. " The Atoll of Diego Garcia and the Coral Formations of 

 the Indian Ocean." By G. C. Bourne, B.A., F.L.S., Fellow 

 of New College and Assistant to the Linacre Professor in 

 the University of Oxford. Communicated by Professor 

 E. Ray Lankester, F.R.S. Received March 12, 1888. 



[Plate 4.] 



The whole of the following paper was planned and a great part of it 

 was written when Captain Wharton's letter appeared in ' Nature,' on 

 Feh. 23. Captain Wharton has anticipated my objections to the 

 theory put forward by Mr. Murray in explanation of the formation of 

 atolls and barrier reefs, and has suggested that the growth of corals 

 on the periphery of a submerged bank is sufficient to explain the 

 elevated rim of a barrier reef or atoll, and the contained lagoon 

 channel or lagoon. In this I cordially agree with him, and can only 

 express my satisfaction that so eminent an observer should have 

 arrived, after an extended study, at conclusions nearly identical 

 with mine. In accounting for the luxuriance of coral growth upon 

 the peripheral slopes of an atoll, I differ slightly from Captain 

 Wharton, and the publication of his letter has led me to extend and 

 modify the plan of the latter half of this paper, in order to show more 

 clearly the points in which I agree with and those in which I differ 

 from him. I may take this opportunity of expressing my thanks to 

 Captain Wharton for his kindness in sending me notes on the struc- 

 ture of the Cosmoledo and Farquhar groups, and to Dr. S. J. Hickson 

 who has given me the benefit of his experience in N. Celebes. 



