34 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
reef are quite deep, with the exception of that part of the lagoon 
which lies south of Yambu and Yanutha and between Thombia and 
the northwestern edge of the outer reef. The deepest part of the 
lagoon is 47 fathoms and the average depth is between 35 and 40 
fathoms. 
In addition to Thombia, which is on the northern horn of Budd’s Reef, 
there are the islands of Yanutha, Yambu, Mungaiwa, Tai ni Mbeka, and 
Rara ni Tinka, which are in the central part of the lagoon. South of the 
central islands the lagoon is also studded with rocks, as well as in 
the southwestern horn of the lagoon. The islands and islets and rocks, ~ 
as well as many of the patches, are of volcanic origin. Yanutha, the 
largest of the islands, about a mile long and half a mile wide, rises to a 
height of 480 feet. It is connected by a coral reef with Mungaiwa Island 
and Mbeka Rock. 
The most interesting of the islands is Thombia (Plate 70), the crater 
of an extinct volcano, having an exterior circumference of about two 
miles. The crater is half a mile in diam- 
eter, with a greatest depth of twenty- 
four fathoms. The rim of the crater 
rises at its highest point in a dome of 
nearly 600 feet. On the northeast side 
WESTERN END OF THOMBIA. the horns of the rim are connected by a 
flourishing fringing coral reef about a 
fifth of a mile in length, the extension on the ridge connecting the horns 
of the fringing reef surrounding the island. Both the inner and outer 
slopes of Thombia are steep, and, except on the northwest side, we find 
over thirty fathoms within a short distance of the shore. 
One cannot fail, on seeing the coral reef growing on the denuded rim 
of Thombia with the enclosed deep lagoon having a depth of twenty-four 
fathoms, to revert to the old opinion that some of the lagoons of atolls 
represented the rim of extinct craters. There is, it seems to me, nothing 
unreasonable in the suggestion that many of the small round atolls, or 
others perhaps rising from great depths and isolated, represent the de- 
nuded rims of such extinct craters as Thombia, or it may be that, if of 
greater size, they may represent parts of such larger craters as Totoya, or 
of circular islands with interior Jagoons resembling extinct craters, like the 
Sound of Fulanga. It seems simple to imagine that, when these small 
extinct craters have been levelled down, and corals have obtained a 
footing, they may have formed such atolls as Pitman’s Reef, Motua 
Levu, Motua lai lai, Williamson Reef, Horseshoe Reef, and other similar 
