It may here be remarked that the birds met with on Houtman's Abrolhos, with the exception of one, resembling in shape and colour a small quail,* were known and common on the mainland. The aquatic species were also familiar to us; but the habit of one kind, of a sooty-black colour, generally called noddies, was quite new--that of building their nests, which are constructed of seaweed and contain only one egg, in trees. There were not many varieties of fish, the most abundant being snappers; of those that were rare Lieutenant Emery made faithful sketches. (*Footnote. Haemapodicus scintilans, Gould.) Half a mile west from Slaughter Point we found two caverns similar to that on East Wallaby Island, from which we got three tons of excellent water. APPEARANCE OF THE REEFS. The reefs surrounding this group appeared very much broken; and even at Easter Group we had found them to be not so regular as at Pelsart's. This suggests the idea, which appears to be borne out by all we saw, that the reefs are compact in proportion to the exposed position of the islands; the shelter afforded by Pelsart Group, in fact, did not require the reefs to be so united round the other islands to the north. From the highest part of East Wallaby Island we discovered a patch of land bearing North-West 1/2 North eleven miles. The outer reef extended in that direction from the south-west point of West Wallaby Island, though it could only be traced by detached patches of breakers. To the south-east of its commencement lies Evening Reef. The observations were made on the north end of the north-east Pigeon Island, bearing West by South half a mile from our anchorage, in latitude 32 degrees 27 minutes 21 seconds South and longitude 2 degrees 1 minute 10 seconds West of Swan River, variation 4 degrees 10 minutes westerly. The temperature of Houtman's Abrolhos is rendered equable by the fact that they lie at the limit of the land breezes; during the month we were there the thermometer averaged 71 degrees. Our protracted stay enabled us to get a tolerable series of tidal observations, which present some singular results. The time of high-water at the full and change was six o'clock when the tide rose 30 inches. It appeared that during the night there was a short flood of six hours with a rise of seven inches, and an ebb of two hours with a fall of only five inches; but that during the day the flow and ebb were nearly equal, the former being eight hours and twenty minutes, the latter eight hours and five minutes, and the rise and fall in each being 25 and 26 inches respectively. TIDAL OBSERVATIONS.