76 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



low islets arranged iu a somewhat circular line, formed by 3 deep bays 

 and spurs from the outer line of islets, as so frequently occurs on wide 

 reef flats in atolls of the Pacific. 



This part of the reef is called Tokorua. This reef flat shelves very 

 gradually from 3^ to 4 fathoms on the west face to 7, and connects witli 

 the " plateau " upon which stand Tara Vai and Aga-kanitai. From 

 Tokorua the reef extends in an indefinite narrow ridge 8 miles long, with 

 from 3 to 8 fathoms, in a southeasterly direction. The western edge is 

 steep to, and the eastern face passes gradually into tlie lagoon, which at 

 that point has a general depth of 8 to 20 fathoms ; the deepest part of 

 this region being at the foot of Mt. Mokoto between it and Tara Vai, 

 though Tara Vai is united with Manga Reva Island by a plateau varying 

 in depth from 3i to 4^ fathoms. 



At the southeastern point of the reef it passes into a wide pla- 

 teau with from 9 to 10 or 15 fathoms. The plateau is about 9 miles 

 wide southwest of Tekava. Tliat part of the atoll has not been well 

 surveyed, so that the position of the reef flat has not been ascertained 

 further west on that part of the east face ; but the southeast passage 

 indicates 5^, 6, and 6^ fathoms where it probably marks the south- 

 western extension of the eastern barrier reef, separating the lagoon from 

 the southern plateau to the south of the encircling reef. 



The western faces of Manga Reva and of Tara Vai are indented by 

 deep bays, formed by spurs running from the central ridge of these 

 islands, the remnants probably of small craters which flanked the 

 large crater, of which Manga Reva forms the western rim and Au Kena 

 is the remnant of the southeastern edge, the former extension of this 

 rim being indicated by the spits uniting the base of Mt. Duff with 

 Au Kena, and by the projection of Au Kena towards the outer barrier 

 reef, and by the numerous patches of coral reef off the northeast point of 

 Manga Reva towards the outer line of Motus till tliey almost unite with 

 the barrier reef. 



The whole of the western bays of INIanga Reva Island are filled by 

 fringing reefs which leave but here and there a deeper pass to the shore. 

 The south face at the foot of the blufl" of Mt. Mokoto and Mt. Dufl" is 

 edged by a flourishing, fringing reef extending nearly half a mile on the 

 plateau at their base. The port of Rikitea is a reef harbor formed 

 within the large fringing reef which occupies the whole of the southern 

 bay of Manga Reva Island. The east face of Tara Vai and part of the 

 east and of the west face of Aga-kanitai are also fringed by reefs. 



The islets and islands of Aka Maru, Mekiro, and Maka-pu are within 



