Fauna of the Reefs — We proceed Homeward. 243 



were also captured. On opening a couple of clam-shells some 

 curious commensal crabs were found Inside, two in each shell ; 

 they seemed to be very reluctant to leave their old quarters. 



Fringing the adjoining shore of the mainland was a large 

 extent of reef covered with fine sand, and bearing a luxuriant 

 crop of short green sea-wracks {Zosterd). Under shelter of this 

 weed we found a great number and variety of Holothurians. A 

 long Synapta, unpleasant to handle on account of the pricking 

 sensation which its spicules imparted to the skin, was here obtained, 

 and a small conger eel dwelt in burrows projecting downwards 

 from the bottom of small pits in the sand, where it might be seen 

 protruding its head on the look-out for its prey. 



The most frequent shell on this part of the reef was a species 

 of Pinna about eight inches long, which, in numbers of three or 

 four together, was generally to be seen anchored vertically in the 

 sand by means of its long byssus. The lips of the shell were so 

 very fragile, and the byssal attachment was so firm, that it was no 

 easy matter to root up an uninjured specimen. About the roots 

 of the green sea-wrack nestled in great numbers a tiny cowrie, the 

 collection, of which gives occupation to great numbers of women 

 and children, who may be seen scattered over the reefs every day 

 at low tide. Great quantities of these shells are exported to the 

 west coast of Africa for the benefit of the negro tribes there, who 

 still make use of them as the current coin of the country. 



With our arrival at Mozambique terminated the surveying 

 operations of the Alert, so that it only remained for us now, on 

 receiving our mails, to make the best of our way to England. \Vc 

 accordingly sailed on the 22nd of May, and proceeded on our 

 voyage towards the Cape of Good Hope, which we expected to 

 reach in about a fortnight's time. However, the weather proved 

 to be most unfavourable, for on passing to the southward of 

 Madagascar, where we had calculated on meeting with the south- 

 easterly trade wind, we encountered instead a westerly breeze, 



