18 J. STANLEY GARDINER. 



26 miles long bj- 10 broad, rim varying with 6 to 12/. of water, very imperfect to the south, 

 inside a shallow basin at about 20 f. with a few soundings up to 24/ Egniont atoll lies between 

 Great Chagos reef and Pitt bank ; it is perfect in shape, 5^ miles long by 2 broad, with only 

 one passage to the north, six islands to the south, lagoon 10 /. deep. S. by E. from Great 

 Chagos lies Diego Garcia, another atoll, completely surrounded by land to the south, but 

 with three passages (3^, 3^, and 6 / of water) to the north; it is 13 miles long by (3 miles 

 in greatest breadth, lagoon maximum depth 19 /, much shoaler to the south. This atoll 

 was visited in 1885 by Mr G. C. Bourne, who subsequently gave an account of it. He came 

 to the conclusion that the whole atoll had been raised a few feet'. 



The Great Chagos bank is of somewhat irregular shape, about 82 miles east and west 

 by 70 miles along a line at right angles. It has one island to the north, described as rocky, 

 and seven to the west. The reef can nowhere be described as awash, save immediately in 

 the neighbourhood of some of the islands, but the line all round the bank is markedly 

 shallow, forming a reef. The general depth of the latter west and north is about 6 /, 

 least 4 /, while east and south it is about 10 /., least 6 /. The lagoon basin has an average 

 depth of about 44 /., greatest depth in centre 48 / ; it is connected to the sea by twenty- 

 three passages. The shallow, encircling reefs are between the passages, great broad flats, sloping 

 in from their outer edges to a depth of 20 /. ; some are obviously small atolls, having 

 shallower water at the edges and deeper in the centre. Against the lagoon basin the 20, 

 30 and 40 / lines are practically conterminous, the reefs down to 40 / being as pre- 

 cipitous in their slope on this side as towards the deep sea. About sixteen shoals arise 

 in the lagoon from the bottom in 40 /. to within 6 to 10 / of the surface, a sti'iking uni- 

 formity in depth with the rim. Although the atoll from the soundings would appear to 

 have been well surveyed it is remarkable that there are in the lagoon no shoals between 

 10 and 40 /. in depth. Of the passages five have about 40 /. of water right through, while 

 six others appear to have over 30 /. 



North of Great Chagos the Victory bank, 4 miles long by 2^ broad, has a perfect rim 

 at 3 /. with 18 /. of water in the centre. Peros Banhos is a perfect atoll with 27 islands, 

 in shape nearly a square 13 miles across ; the greater part of its reef is awash save to 

 the S.E., where it is submerged by about 4 f. of water, but even here two islets rise above 

 the surface. The lagoon has thirteen passages to the exterior and is much studded with 

 shoals; its general depth is less than 30/., but one sounding of 41/ is recorded. The whole 

 atoll bears a very close comparison to some of the more southern ones in the Maldives. 



' P. R. S., vol. 43, pp. 440 — 461, 1888. Mr Bourne has this publication the crabs of this genus do not construct 



informed me that masses of the conglomerate (or reef-rock) burrows lined with weed, and one species lives above tidal 



exist everywhere on the outer reef in pinnacles and buttresses, limits; the holes of shore forms cave in, and are made 



extending out from the beach between tide-marks. The afresh after every rise of tide, not necessarily too in the 



stratiticatiou of these is stated to be horizontal. No coral- same place. Mr Bourne's deductions on lagoon formation 



rock is built up thus above the low tide limit, so that these in general by solution do not seem to me to carry much 



absolutely prove a change of level. Mr Bourne in his account weight. They are based on experience of one atoll alone 



bases his views rather on the horizontal stratification of and that of abnormal form, in that it is of considerable size 



♦' shingle rock," an agglomeration of broken corals, mollusc but completely surrounded with land except to the north ; 



and echinoderm shells, etc. ; this rock was probably originally the circulation of water in the south half of the lagoon, 



a formation on the lagoon side of a reef awash, and it would which is especially referred to, cannot be anything but 



be interesting to know more of its distribution. The observa- inconsiderable as compared even to any atoll with reef only 



tions on Ocypode I cannot admit as referring to a crab of this awash in several positions, 

 genus at all; as Mr Borradaile points out in his paper in 



