The Porpoises differ from the Dolphins in having their snout short and uniformly rounded, without a beak-like projection. Their teeth are compressed, sharp-edged, and rounded, their number from twenty-two to twenty-five in each jaw. Their skin is smooth and shining, black above and white below, and as they never attain a greater length than four or five feet from the tip of the muzzle to the extremity of their flat horizontal tail, they may be regarded as the smallest of the Cetacean Order. These animals abound in every sea, and many people have witnessed their unwieldy gambollings, the character of which is by no means badly expressed by their name (porc-poisson, hog-fishes). They have, in fact, somewhat the appearance of floating pigs, as they wallow in the trough of the sea and roll over each other amid the foaming waves. Their food consists entirely of Fishes, of which they destroy great quantities. They follow the shoals of Herrings and of Mackerel, and when pursuing their prey, not unfrequently venture into the estuaries of rivers, and make excursions up the rivers themselves. THE NARWHALS. [Illustration: NARWHAL.] The Narwhals have no teeth, but are furnished with an enormous tusk, that projects from the upper jaw, and becomes a most formidable weapon. The Narwhal is an inhabitant of the Arctic seas, where it sometimes attains a length of from twenty to twenty-five feet. Its skin is beautifully marbled with brown and white; its muzzle is round, and its mouth, unlike that of other Cetaceans, is disproportionately small. Its single tooth, or horn-like tusk, projects from the head in a line with the body, sometimes to the length of nine or ten feet. It is spirally twisted, tapering to a point, and as it is composed of the hardest ivory, is capable not only of transfixing the body of a Whale, but when impelled by such momentum as is derived from the speed of its ponderous owner, has been known to penetrate the oaken ribs of a British man-of-war to the depth of nearly a couple of feet, and probably has thus caused the loss of many ships incapable of resisting the shock. HERBIVOROUS CETACEANS. Until a very recent period the animals composing this family were quite unknown, or perhaps we ought rather to say they were just sufficiently known to make them the objects of superstition. Seeing that there is in their general appearance, somewhat of a resemblance to the human form, the casual glimpses obtained of them at once satisfied their first discoverers that they were Tritons and Sirens, such as they had read of in mythological writings, and the belief in the existence of Mermaids and Mermen was thus at once confirmed. In the works of Gesner, Aldrovandus and Jonston, the earliest authors after the renaissance of Natural History in modern times, the figures of creatures having human bodies joined with the tails of Fishes are inserted with the utmost faith in their existence.