60 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
islands as Namuka, Mango, Ngele Levu, Fulanga, Oneata, Ongea, and 
others. It is quite probable that the islands and islets of this group 
represent the remnants of the submarine erosion and denudation which 
have cut the greater island of Yangasa, consisting of elevated limestone, 
which once occupied the area of the lagoon, into smaller islands by a 
process similar to that now going on to cut up the island of Navutui- 
loma, —a process which has left, as indicating its former greater extent, 
the islands of the cluster, ‘its rocks and islets, and the numberless heads 
cropping out everywhere over the outer reef flats, and in the extension 
of the spits and points of its islands. 
To the eastward of the Yangasa cluster, and separated from it by a 
narrow channel of about half a mile in width and a depth of a little 
over 100 fathoms, lies Thakau Levu (Plate 22), an elongated horseshoe- 
shaped atoll, nearly four and a half miles in length, with a greatest 
breadth of less than two miles. The lagoon enclosed by the outer reef 
flat is open on the west face. The greatest depth in the lagoon is 11 
fathoms; it is studded with heads and patches. The reef flat at the 
eastern point of the lagoon is fully a mile wide. 
South of Thakau Levu (Plate 22) lies the flat reef of Thakau Thi- 
kondua, and the small island Naiabo, of elevated limestone, rising to a 
height of forty feet, surrounded by a narrow encircling reef enclosing 
a shallow lagoon. Still farther south are the flat reefs of Thakau Reiva- 
reiva and Thakau Nasokesoke, both, according to the sailing direc- 
tions, dry at low water and steep to all round. We did not visit either 
Thakau Levu or the last mentioned reefs. 
Ongea. 
Plates 22, 22°, Figs. 6, 7, and Plate 94. 
The Ongea group consists of two large islands, Ongea Levu and 
Ongea Ndriti, and numberless mushroom-shaped or conical or dome- 
shaped islets and rocks, studding both the passage between the two 
principal islands or fringing the shores or extending across the openings 
of the bays which indent the coast line of Ongea Levu and Ongea Ndriti 
(Plate 22). The larger island, Ongea Levu, runs nearly north and 
south ; it is four miles long, and varies from one to two miles in width ; 
it rises to a height of 270 feet. This island lies in the centre of the north- 
ern part of the lagoon, about equidistant from the outer encircling reef 
flats, within a channel from three quarters of a mile to a mile in width, 
and varying in depth from seven to twelve fathoms. Off the southern 
