_Sterna macrotarsa_, Gould in Proc. of Zool. Soc., Part V. p. 26; and in Syn. Birds of Australia, Part II. Crown of the head and back of the neck black; all the upper surface and primaries light silvery-grey; remainder of the plumage white; bill and feet black. As I did not meet with this bird myself either in Van Diemen’s Land or in any other of the Australian regions, I have not figured it. Genus GYGIS, _Wagl._ One species of this Polynesian genus of Terns is found in Australia. 569. Gygis candida Vol. VII. Pl. 30. Genus HYDROCHELIDON, _Boie_. The value of minor genera or subgenera, as naturalists may choose to designate them, is much strengthened, when species, which have been assigned to either of them from countries so distant from each other as Australia and Europe, are found to possess similar habits, but differing from those of the other members of the family. Thus the members of the present little group inhabit the inland waters and marshes of both countries; make their nests among the rushes, and lay thickly-marked eggs, in both of which particulars they differ from the other Terns; the generality of which deposit their eggs on the shingles of the sea-shore, while others, the _Gygis candida_ for instance, lay their single egg on the horizontal branch of a tree, so totally unprotected, that how it is retained in its position during windy weather is a perfect mystery; others again, such as the Noddies, bring together large masses of sea-weed, which they either pile upon the swinging branch of a Mangrove or on the jutting point of a rock. All these facts should be studied by ornithologists before they discard subgenera proposed by their fellow-labourers, and replace the species they may have so divided in the genera of the older writers, who must necessarily have known less of the subject; for wherever a difference occurs in the habits of the members of any great family a variation more or less marked will be found in their structure. So far as my own observations go, and they have not been few, if I have read the great book of nature aright, the genera, instead of being reduced, might with propriety be multiplied without the risk of our being burthened with a genus for every species, as some writers affect to fear would then be the case. 570. Hydrochelidon fluviatilis, _Gould_ Vol. VII. Pl. 31. A fine marsh Tern differing from its European prototypes _H. nigra_, _H. leucoptera_ and _H. leucopareia_. Genus ONYCHOPRION, _Wagl._ Of this form two species frequent the Australian seas. 571. Onychoprion fuliginosus Vol. VII. Pl. 32.