EONGELAB. 305 



stantly shifting in these atolls. It is interesting to note the marlied 

 difference there is in the method of handlino- the material in the large 

 lagoons, due to the reach of a very strong northeast trade or that of the 

 more moderate southeast wind. In fact, the action of the northeast trade 

 on the eastern lagoon face of the west rim of the Marshall Island atolls 

 is nearly as great as that of the southeast trades on the eastern face of the 

 Paumotus or of some of the atolls to the south of the line. 



One of the most characteristic corals we found in the Marshall Islands 

 is a long, slender branched Madrepore which grows in smooth water in 

 protected parts of the lagoon. It would be an interesting study to compare 

 the modifications of different species of corals flourishing best in the midst 

 of breakers, or when found in shallow or deep water along protected shores. 

 The variation we have observed in some of the corals is so great that at 

 first glance specimens taken from different parts of the same atoll would 

 hardly seem to belong to the same genus. 



One of the best examples of such adaptation, due perhaps to a constant 

 struggle against the action of the heavy breakers to which they are sub- 

 ject, is seen in the peculiar growth of the spines of Heterocentrotus and of 

 Colobocentrotus. The spines in the one case are hiassive clubs with which 

 the sea urchin braces itself in corners or angles of the reef flats and retains 

 its hold against the action of the sea; while in the other genus the immense 

 development of the suckers on the lower side and the reduction of the 

 spines to a dome-shaped pavement gives to that genus a still greater 

 security. It is comparatively rare to find the tests of these species thrown 

 up on the beach flat or on the shingle beaches. Side by side with them in 

 pools exposed to the full action of the sea are found immense numbers of 

 Holothurians; they seem to have no capacity for protecting themselves 

 against this very action, and yet do not appear to be affected by the 

 action of the breakers. 



The section of the beach we examined (PI. 172, fig. 1) shows a high, steep 

 beach consisting of fine white coral sand of about twelve feet in heiijht, 

 flanked at the base by a small secondary beach of coral shingle covered 

 on the top by large blocks of beach rock conglomerate, thrown by the sea 

 towards the lagoon side. 



20 



