A SURVEY SEASON IN THE NICOBAR ISLANDS. 
987 
among the green ■weed we find green erabs and mottled green gobies. In places 
this sandy or mud flat is, instead of being smooth of surface, thro\ni into a 
number of little valleys and ridges : at the bottom of each valley a funnel-shaped 
depression loads doum to a small central burrow and sheltering in the mouth of 
this is a small gobioid fish, that on the first sign of danger disappears down tho 
hole. On the stretch of sand that is laid bare by each receding tide are hosts of 
small crabs, which live in burrows : some of these are round and spherical, 
Dotilla, while others have one claw enormously enlarged, Gelasimus. Sometimes 
a whole stretch of sandy beach may be twinkling with little brightly-coloured 
objects as these fiddler-crabs wander over the surface in search of food, each 
male waving its enormous gaudy-coloured claw : but the first step fonvard or 
even a sudden movement of the arm will cause the total disapix^arance of these 
little animals douii their holes. 
A /jingle example of the cel, Ophichthys coluhrinus, was found lying buried in 
the siand, with only the anterior two or three inches protruding, on the beach near 
the entrance to Expedition Harbour. Its black and white rings render it very 
conspicuous and may be an example of warning colouration, for it has been 
credited with possessing poisonous properties.* Other inhabitants of these 
stretches of sand and mud are starfish, of which a few were obtained near Xaval 
Point, and holothurians though these latter are far more common in rock pools 
and on the reefs themselves. Crawling over the mud-flats at low tide are numbers 
of molluscs, Nassa and Nntica being the genera more commonly represented, 
and numerous examples of the stalk-eyed crabs of the genus Ocypoda can be 
found busily hunting for food. In certain areas Gelasimus crabs are also common 
but for some unexplained reason their distribution seems to be discontinuous, 
large areas of sand showing no trace of either them or their burrows. ** 
An interesting feature of the reefs is the peculiarly local distribution of the 
various kinds of coral. Everywhere the main bulk of the wall of coral on the 
face of the reef consists of Porites but inside this in any one area some one 
particular kind of coral seems to thrive to the almost complete exclusion of all 
other forms. Commencing off Xaval Point at the eastern entrance to Xankauri 
Harbour and extending northward along the east coast of Camorta the bulk 
of the reef consists almost entirely of stag-horn coral {Madrepora), scattered 
about among the branches of which are numbers of round or oval mushroom 
corals (Fitngcoi^ and in patches a few colonies of the branchmg forms of Mille- 
pora and PociP.opora. The prevailing tint of the growuig coral is brown or red 
and the tip? of the branches vary in colour but are usually pale blue or mauve. 
Living amoiig the branches of the coral and firmly attached to them are numer- 
ous small flattened oysters and crawling over the branches are crabs and alpheids 
many of which show very distinct protective coloimation. If a mass of this 
stag-hom coral be lifted out of the water and shaken over a net a number of small 
fish, belonging for the most part to two species, will be secured. These are the 
little greenish-yelloAv Heliasfes lepidurus and the black and white banded Tetra- 
drachmum aruanum. 
On the opposite side of the entrance to Xankauri Harbour off Reed Point the 
bulk of the coral growing on the reef consists of rounded masses of Moniipora in 
which the plates are arranged vertically and rarliate from the centre of the mass. 
The basic colour to these colonies also is browi but the margin of each plate is 
pure white. Here we found, embedded in and almost completely enclosed by 
coral debris, a number of Tridacna, whose brilliantly coloured mantle renders 
* Wood, Jones., “ Coral and Atolls,” p. 329, London, 1912. 
** In many places trees that were growing on the edge of the jungle have fallen 
down and their dead and decaying trunks and branches now lie prone on the 
sandy shore at or a little below high-watermark and these serve as a habitat for 
numbers of molluscs belonging to two species of Littorina. 
