But little difference of opinion exists among naturalists as to the limits of our last order; indeed, a swimming bird is at once recognisable by characters so sharply defined that there is scarcely room for doubt or hesitation. The grand character whereby they are all distinguishable is to be found in the structure of their feet, the toes of which are more or less completely united to each other by a broad web by which they are converted into paddles, admirably adapted to propel them through their appropriate element. Hence they swim with facility, and frequent rivers, lakes, and seas. Generally speaking, the Natatores move awkwardly on land, as, in order to render the strokes of their paddle-like feet more effective, their legs are placed more or less behind the centre of gravity, so that when on the ground they have, in a great measure, to retain their balance by muscular exertion, as may be seen in the Goose or Swan as they waddle clumsily over the grass. In some cases these birds are compelled to assume an upright attitude, as do the Auks and Penguins. In the structure of the feet important differences are observable, generally only the three front toes are connected together by the web, but occasionally the hind toe is likewise included in the expansion of the skin, or is provided with lateral appendages that materially enlarge its surface. The form of the body in the swimming birds is boat-like; in those that dive, the ribs are strong and carried back, so as to almost entirely surround the enclosed viscera, and thus defend them from undue pressure. Their plumage is dense, close, and waterproof, consisting of a thick under garment of down overlaid with broad flat feathers, which throw off the water as if their surface were oiled. In many groups the neck is very long and the tail very short, while in others the tail composed of rigid quills is employed as a rudder when the birds are diving. Some are utterly incapable of flight, and use their wings in the water as though they were fins. The members of this order live almost exclusively upon animal food, only a few preferring a vegetable diet. All, without exception, are sociable in their habits; most of them exceedingly prolific, some species, however, lay but one egg. Frequently they crowd their favourite breeding-places in great numbers, and are devotedly attached to their offspring; some will even incubate eggs, and rear young that are not their own.

THE SIEVE BEAKS.

THE Sieve Beaks (Lamellirostres), according to our usual plan, are placed at the head of the Natatorial order, as in these Tooth-billed and Soft-billed Swimmers, as they have been called by various writers, the peculiar attributes of Swimming Birds are most equally developed, and their locomotive capabilities most varied. Whoever examines a Duck with attention, will appreciate the leading features whereby the Lamellirostres are distinguished. Their bill is generally depressed, broad, and always laminated at the sides; the lamination being more prominent in some species than in others, but always so arranged as to form a sieve-like apparatus, by means of which these birds obtain their food, in a manner peculiar to themselves. This remarkably constructed beak, which is seldom longer than the head, is usually straight, broad, and flatly arched above, terminating in front in a broad nail-like appendage, while at the sides it presents a series of horny laminæ, which interlock with similar developments from the under jaw. With the exception of the margins, which are tolerably hard, the entire beak is covered with a soft skin, and is largely supplied with branches of the fifth pair of nerves,