The swifts and bats--the latter about the size of the ordinary English bat--avail themselves of the shelter afforded by the caves without incommoding one another, for, by a sort of Box and Cox arrangement, the former occupy the caves during the night and the latter by day. Standing at the Simud Putih entrance about 5 P. M., the visitor will suddenly hear a whirring sound from below, which is caused by the myriads of bats issuing, for their nocturnal banquet, from the Simud Itam caves, through the wide open space that has been described. They come out in a regularly ascending continuous spiral or corkscrew coil, revolving from left to right in a very rapid and regular manner. When the top of the spiral coil reaches a certain height, a colony of bats breaks off, and continuing to revolve in a well kept ring from left to right gradually ascends higher and higher, until all of a sudden the whole detachment dashes off in the direction of the sea, towards the mangrove swamps and the _nipas_. Sometimes these detached colonies reverse the direction of their revolutions after leaving the main body, and, instead of from left to right, revolve from right to left. Some of them continue for a long time revolving in a circle, and attain a great height before darting off in quest of food, while others make up their minds more expeditiously, after a few revolutions. Amongst the bats, three white ones were, on the occasion of my visit, very conspicuous, and our followers styled them the Raja, his wife and child. Hawks and sea-eagles are quickly attracted to the spot, but only hover on the outskirts of the revolving coil, occasionally snapping up a prize. I also noticed several hornbills, but they appeared to have been only attracted by curiosity. Mr. BAMPFYLDE informed me that, on a previous visit, he had seen a large green snake settled on an overhanging branch near which the bats passed and that occasionally he managed to secure a victim. I timed the bats and found that they took almost exactly fifty minutes to come out of the caves, a thick stream of them issuing all that time and at a great pace, and the reader can endeavour to form for himself some idea of their vast numbers. They had all got out by ten minutes to six in the evening, and at about six o'clock the swifts began to come home to roost. They came in in detached, independent parties, and I found it impossible to time them, as some of them kept very late hours. I slept in the Simud Putih cave on this occasion, and found that next morning the bats returned about 5 A.M., and that the swifts went out an hour afterwards. As shewing the mode of formation of these caves, I may add that I noticed, imbedded in a boulder of rock in the upper caves, two pieces of coral and several fossil marine shells, bivalves and others.