50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
patches, but nearly the whole of the west face until it joins the western 
spur of Nanuku Reef 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 Nanuku Reef. 
The northern islet, Nanuku 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 Levu, 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 exteuding 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 charts 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 Reefs 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 
Nanuku lai lai and Nanuku Levu islets are the solitary remnants, while 
Nukusemanu 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 Reef 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. 
