DIFFERING from the last order (Gruiformes), with which in a number of particulars they seem to be most closely related, by the technical characters of opisthocoelous dorsal vertebrae and a four-notched instead of two-notched sternum, is this vast cosmopolitan order of so-called Plover-like birds. But as previously indicated, the selection of a popular name for a great group that will convey anything like an adequate impression of all the forms included in it is well-nigh impossible, and especially so in the present case, for what are taken to be the central or typical forms may be indeed are very unlike the outliers that are bound to them by a greater or less number of often minute or obscure but relatively important characters from the standpoint of classification. The typical Plover-like birds, or what Gadow calls the Charadriiformes in the narrow sense, are mostly Rail-like marsh or shore birds, with rather long necks, slender, long legs, and long, slender bills, which may be approximately straight, curved upward or downward, or, exceptionally, bent sideways, usually short wings and tails, and a dull-colored, often streaked, plumage. From these we pass to the aquatic, mainly oceanic, Gulls, Terns, Skimmers, and Auks and their immediate allies, and come by way of the somewhat anomalous Sand Grouse to the great and quite diversified group of Pigeons. The latter are in no sense aquatic and at first blush seem very unlike those first alluded to, and were it not for the pronouncement of comparative anatomy, we might be justified on mere external appearance in separating them quite widely, and yet the thread of apparent, though perhaps remote, kinship seems to run through them all.
The present order is divided into four suborders : the Limicolce, embracing six families, among them the Plovers, Snipes, Pratincoles, Thick-knees, and Jacanas; the Lari, including the families Laridce, or Gulls, Terns, and Skimmers, and the Alcidce, or Auks; the Pterocles, which includes the single family Pteroclidce, or Sand Grouse; the Columbce, or Pigeons, divided among three families, the Dididce for the Dodo and Solitare, the Columbidce for the great majority of Pigeons, and the Didunculidce for the peculiar Tooth-billed Pigeons. The first two suborders are brigaded together under the name of Laro-Limicolce, and the others into the Pteroclo-Columbce, in accordance with their affinities. The characters of the various groups are set forth under the several headings.