896 
Proceedings of the Royal Society 
a phenomenon is unusual in the Pacific, where, T on account of the 
greater rapidity of the submarine slope, the rollers dash at once 
against the face of the reef. Professor Dana, who regards this 
process of organic degradation as inadequate, would attribute the 
formation of the chalky mud of lagoons and reef- flats to self- 
trituration of the gentlest kind possible. 
Yery fine sand may he distributed over the bottom of lagoons and 
lagoon-channels in the following manner : — I have often observed 
this material floating in the shallower places on the calm surface of 
the water, the particles being connected together and coated by some 
oily film, such as invests the pellets of sand voided by holothurians 
and other animals that frequent coral reefs. A touch of the finger, 
by wetting the upper surfaces of the particles, causes them to sink. 
This material appears to be taken up by the water as it runs gently off 
the sandy flats that are exposed by the ebbing tide. In this manner, 
during a calm day, sand may be transported on the surface of the 
water from the margin to the centre of a lagoon, when some slight 
disturbance, such as a cat’s paw of wind, will send it to the bottom. 
I may here refer to the important part which echinoids take in 
the degradation of coral rock. On every reef it will be noticed that 
the blocks of dead coral, and especially the flattened dead summits 
of the massive corals, such as Porites , display singular narrow and 
somewhat tortuous grooves, 8 to 12 inches long, 1 to 2 inches wide, 
and 3 to 4 inches deep.* The sides of these grooves are worn 
smooth and are lined by a pink nullipore. Nestled at the bottom 
of each is to be found a solitary echinoid belonging to a species of 
Rchinometra , the size of which exactly corresponds with its furrow, 
so that it is difficult to dislodge it without injuring the animal. 
That these grooves are the permanent homes of these echinoids is 
shown in the character and arrangement of the spines on the two 
sides of the tests, which are in close apposition with the walls of the 
furrow. The spines are here small and partially stunted, and no 
longer radiate from the animal, but are pressed back against the 
test. These grooves are evidently produced by the occupant, for it 
is well known that echinoids are able to gouge holes for themselves 
in the solid rock. According to Professor A. Agassiz, species of 
* These grooves are often preserved on flats of coral rock that have been 
recently elevated a few feet or more above the sea. 
