January 19, 1900.] 



SCIENCE. 



97 



which grew madrepores. The bottom of 

 the lagoon is covered by Tridacna and 

 masses of a species of Area live near the 

 edge, the intervening spaces being tilled 

 with nullipores. The entrance to the la- 

 goon is perhaps 150 feet wide, and there is 

 a cut through the beach rock covering the 

 old ledge, giving access to the sea into the 

 lagoon at certain stages of the tide. The 

 water in the lagoon is quite warm. 



At Pinaki, as at other atolls and islets to 

 the eastward, there are fewer cocoanuts 

 than on the westward atolls, and the vege- 

 tation consists in great part of pandanus 

 and puteau trees and the usual coral reef 

 vegetation of the Paumotus and Fijis. 



The only atoll we have seen, the lagoon 

 of which is entirely shut oif from the sea, is 

 Niau. In this case the old ledge forming 

 the rim of the land, which surrounds the 

 nearly circular lagoon, is about a third of a 

 mile in width, and sufficiently high, 15 to 

 20 feet, to prevent any sea from having ac- 

 cess to it except in case of a cyclone, as 

 that of 1878, when the sea washed into the 

 lagoon. The lagoon is shallow, of an aver- 

 age depth of about three fathoms, the deeper 

 parts perhaps five. The water is brackish, of 

 a density of 1.0216 at 28 degrees C. There 

 are no corals living in it, but a species of 

 mullet is found, as well as many marine 

 shells, which, like those in the lagoons of 

 San Salvador, in the Bahamas, are of di- 

 minutive size compared to their representa- 

 tives living outside. The floor of the 

 lagoon is covered with algiB. The lagoon 

 has probably a slight connection with the 

 sea, the water percolating through the 

 limestone ring separating it from the outer 

 reef flat. It is verj' difBcult in this case to 

 decide whether this lagoon has been grad- 

 ually filled up after elevation or whether it 

 is merely a sink on a more or less uneven 

 limestone surface. 



Dana and the other writers on coral 

 reefs mention a great number of lagoons 



as being absolutely shut off from the sea. 

 I take it these statements are due to their 

 descriptions being taken from charts, many 

 of which (as in the case of the Paumotus) 

 are very defective. For nothing is easier 

 than to pass at a short distance by the wide 

 and narrow cuts which give in so many 

 places the freest access to the sea to the in- 

 terior of the lagoons, and described as closed 

 because they have no boat passages. I 

 could mention as instances of such lagoons 

 those of the atolls of Takume, Hikueru, 

 Anaa, etc., which may be said to be closed, 

 yet into which a huge volume of water is 

 poured at everj^ tide over low parts of the 

 encircling reef flats. 



The character of the coral reefs of the 

 Paumotus is very different from that of 

 other coral reef regions I have seen. No- 

 where have I seen such a small number of 

 genera, so many small species, and such 

 stunted development of the corals. ISTone of 

 the great heads of the genera so character- 

 istic of the West Indian regions, or of the 

 great barrier reef of Australia, are to be seen ; 

 with the exception of a couple of species 

 of alcyonaria they are absent, so far as our 

 experience shows, and there are but few 

 sponges and gorgonians to be found among 

 the corals. The bathymetrical limit of the 

 reef-building corals seems to be about 20 to 

 22 fathoms, but nowhere have I seen such 

 extraordinary development of incrusting 

 nullipores as on the sea edge of the shore 

 platforms of some of the Paumotu atolls, 

 where they build up to a height often four 

 feet to form the outer edge of the secondary 

 barrier reef so frequently seen along the 

 reef faces of the Paumotus. 



On the 4th of November we were well on 

 our way to Mehitia, the easternmost of the 

 Society Islands, the account of which will 

 be included in my next letter giving the 

 results of our examination of the Society 

 Islands. 



We have taken a large number of photo- 



