458 MOLLUSCA OF MAST HEAD REEF, QUEENSLAND, I., 



ashore in as many minutes. Though we did not find their eggs 

 the disturbed surface of the sand showed oviposition. From the 

 ventral surface of the neck of one we killed for food, Mr. Kesteven 

 collected a species of Branchellion. 



The crest of the encircling reef is bared at about a quarter 

 ebb, at which state of the tide it assumes the aspect of an atoll 

 with a shallow lagoon enclosed by a rim of boulders. These dead 

 coral blocks on the summit of the reef, locally called " nigger- 

 heads," stand out against the sky like tombstones in a cemetery. 

 They are composed of an enormous astrean coral, a species which 

 does not occur between tide-marks, but doubtless is washed up 

 by heavy gales from the submarine base of the reef. Once 

 perched, these blocks do not travel. A mass of a cubic yard or 

 so, which two men might overturn, sheltered a host of mollusca, 

 tunicates, sponges and such-like cryptozoic fauna; these were 

 tenants who anticipated a long lease of their abode. But the 

 nigger-heads suffer great and rapid erosion, being worn into 

 cavities like those of a melting mass of ice. Purple rock oysters 

 are common on these blocks, and I noticed that all the aged oysters 

 projected like spurs from the surface to which they were 

 attached by the ventral margin. Originally they were fixed by 

 the umbonal end, and as the first point of attachment wore away, 

 they clung by the newer part of the valve. From the appearance 

 of these oysters I deduce that in four or five years these nigger- 

 heads have lost by erosion and solution a crust two inches thick. 

 The presence of " pinnacles" on Mast Head has already been noted 

 by Prof. A. Agassiz.* He is inclined to regard all such as 

 remnants left from the erosion of a larger, more elevated mass. 

 All nigger-heads which I have examined on the Queensland coast 

 appear to me to be erratics. Had the surf planed down an 

 elevated mass, I question if the last phase of degradation would 

 assume this form. 



As the ebb tide retreats, smaller blocks and broken branches of 

 dead coral are seen strewn along the summit of the reef. Species 



*Bull. Mus. Comp. Zool. xxviii. 1898, p. 105. See also Saville Kent, 

 'Great Barrier Beef,' 1893, pi. xxx.b; and 'Naturalist in Australia,' 1S97, 

 p.143. 



