The common porpoise, when full grown, attains a length of 5 feet or a little more. The dimensions of an adult female specimen from the English Channel were as follows: —length in straight line from nose to median notch between the flukes of the tail, 62 J inches; from the nose to the anterior edge of the dorsal fin, 29 inches; height of dorsal fin, 4∣ inches; length of base of dorsal fin, 8 inches; length of pectoral fin, 9∣ inches; breadth of pectoral fin, 3 J inches; breadth of tail flukes, 13 inches. The head is rounded in front, and differs from that of the true dolphins in not having the snout produced into a distinct “beak” separated from the frontal eminence by a groove. The under jaw projects about half an inch beyond the upper one. The aperture of the mouth is tolerably wide, and is bounded by stiff immobile lips, and curves slightly upwards at the hinder end. The eye is small, and the external ear represented by a minute aperture in the skin, scarcely larger than would be made by the puncture of a pin, situated about 2 inches behind the eye. The dorsal fin is placed near the middle of the back, and is low and triangular. The pectoral fins are of moderate size, and slightly falcate.

The horizontally-expanded caudal fin is of the form common to all Cetacea. The external surface, as in the rest of the order, is smooth, shining, and devoid of hair, though in the foetal condition a few bristles are found near the nose. The upper parts are dark grey, or nearly black, according to the light in which they are viewed, and the state of moisture or otherwise of the skin; the under parts are pure white. The line of demarcation between these colours is not distinct, washes or splashes of grey encroaching upon the white on the sides, and varies somewhat in different individuals. Usually it passes from the throat (the anterior part of which, with the whole of the under jaw, is dark) above the origin of the pectoral fin, along the middle of the flank, and descends again to the middle line before reaching the tail. Both sides of the pectoral and caudal fins are black. The anterior edge of the dorsal fin is often furnished with a row of small rounded horny spines or rather tubercles of very variable number, which have been thought to indicate a specific distinction between the animals possessing them (JPhocaena tυberculifera, Grey) and those without them, but this has not been confirmed by other characters. One of the most characteristic anatomical distinctions between the porpoise and other members of the Delphinidae is the form of the teeth, which (numbering twenty-three to twenty-six on each side of each jaw) instead of'the usual conical, sharp-pointed, recurved shape, all have expanded, flattened, spade-like crowns, with more or less marked vertical grooves, giving a tendency to a bilobed or often trilobed form (see fig. 2).

The porpoise is sociable and gregarious in its habits, being usually seen in small herds, and frequents coasts, bays, and estuaries rather than the open ocean. It is the commonest cetacean in the seas around the British Isles, and not unfrequently ascends the river Thames, having been seen as high up as Richmond; it has also been observed in the Seine at Neuilly, near Paris. It frequents the Scandinavian coasts, entering the Baltic in the summer; and it is found as far north as Baffin’s Bay, and as far west as the coasts of the United States. Southward its range is more limited than that of the common dolphin, as, though very common on the Atlantic coasts of France, it is not known to enter the Mediterranean.

It feeds on fish, such as mackerel, pilchards, and herrings, of which it devours large quantities, and, following the shoals, is often caught by fishermen in the nets along with its prey. In former times it was a common and esteemed article of food in England and in France, but is now rarely if ever eaten, being commercially valuable when caught only for the oil obtained from its blubber. Its skin is sometimes used for leather and boot-thongs, but the so-called “porpoise hides” are generally obtained from a different and larger species of cetacean, the Beluga of the northern seas.

A closely similar if not identical species from the American coast of the North Pacific has been described under the name of Phocaena υomerina, and another from the mouth of the Rio de la Plata as P. spinipennis. Another nearly allied form is N comeris phocoenoides, a small species from the Indian Ocean and Japan, with teeth of the same form as those of the common porpoise, but fewer in number (eighteen to twenty on each side of each jaw) and of larger size, and more distinctly notched or lobed on the free edge. It is distinguished from the common porpoise externally by its entirely black colour and the complete absence of a dorsal fin.