50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



patches, but nearly the whole of the west face until it joins the western 

 spur of Nanuku Eeef consists, like the east face, of a long but narrow 

 reef flat, with from two to seven fathoms, and studded with heads. The 

 greatest depth in the lagoon is 52 fathoms, with an average depth of 

 from 25 to 40 fathoms. The interior of the lagoon, especially the 

 northern part, is studded with heads. 



We examined the islets on the southern spits of the Xanuku Reef. 

 The northern islet, Xanuku lai lai, has been nearly washed away during 

 the hurricane of 1893 (Plate 107). The southern islet is covered with 

 shrubs and cocoanuts (Plate 104). The eastern face of this islet, 

 Nanuku Levn, is flanked with beach rock (Plate 106). There were a 

 large number of negro-heads of beach rock and of elevated coralliferous 

 rock scattered upon the reef flats adjoining the island (Plate 103), and 

 both to the north and towards the southern extremity of the reef flat 

 ridge (Plate 105). Coral heads begin at about seventeen fathoms off" the 

 lee side of the reef, the patches increase in size and number at twelve 

 fathoms, and form a very fine flourishing belt between six to one and a 

 half or two fathoms. "We could trace these coral patches on the nar- 

 row reef flat ridge extending northward. This ridge is at some points so 

 narrow that the breakers form a long white mass of rollers as they fall 

 from the windward to the leeward side of the reef flat. The leeward 

 pitch is not as steep as the chai'ts seem to indicate. We found a hun- 

 dred fathoms on that side, hard bottom, at a distance of one and three 

 quarters miles from the leeward edge of the reef flat, two and a quarter 

 miles northerly from Nanuku lai lai (Plate 18). 



The island which once covered the tract extending from the northern- 

 most horn of the Nukusemanu Eeefs to the southwestern horn of the 

 Nanuku Reefs (Plate 18) was probably flanked by outer ridges of hills 

 running into a long narrow ridge at the southern extremity, of which 

 Xanuku lai lai and Xanuku Levu islets are the solitary remnants, while 

 Xukusemanu Island is the only fragment of the northern extremity of 

 the eastern ridge. There must have been a valley separating the ridges 

 now indicated by the open lagoon, with a greatest depth of over forty-five 

 fathoms, extending from Nanuku Eeef to Nukusemanu Reef, which was 

 cut into at many points all along the outer edges of the ridges, and con- 

 nected the plateau of submarine erosion with the great sound of the 

 interior. 



