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. Tlie 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 Eeef, 

 there are the islands of Yanutha, Yambu, Mungaiwa, Tai ni Mbeka, and 

 Kara 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 wnde, rises to a 

 height of 480 feet. It is connected by a coral reef with Mungaiwa Island 

 and Mbeka Eock. 



The most interesting of the islands is Thombia (Plate 70), the crater 

 of an extinct volcano, having an exterior cii'cumference 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. tliB homs of tlic riui arc 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 lagoons resembling extinct craters, like the 

 Sound of Fulanga. It seems simple to imagine that, when these small 

 extinct ci'aters 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 



