The herds of Bisons wander over the country in search of food, usually led by a bull remarkable for strength and fierceness. While feeding, they are often scattered over a great extent of country; but when they move, they form a dense and almost impenetrable column, which, when once in motion, is scarcely to be impeded. Their line of march is seldom interrupted, even by considerable rivers, across which they swim, without fear or hesitation, nearly in the order in which they traverse the plains. The Bisons which frequent the woody parts of the country form smaller herds than those which roam over the plains, but are said to be individually of a greater size. The rutting takes place the latter part of July and the beginning of August, after which the cows separate from the bulls in distinct herds. They bring forth their young in April: from which it appears that the term of gestation is about nine months. The pair of American Bisons in the Zoological Gardens produced a calf in 1849; from the observations made in that instance, the period of gestation was calculated at 270 days. The most important anatomical difference between the American and the European is, that the American has fifteen pairs of ribs, whereas the European has but fourteen. The following are the dimensions of a large specimen:-- Ft. In. From the nose to the insertion of the tail 8 6 Height at the shoulder 6 0 " at the croup 5 0 Length of the head 2 1 Their weights vary from 1200 to 2000 pounds. [Illustration: Head of young male Bison.] THE AUROCHS, OR EUROPEAN BISON. _Bos Bison._ [Illustration] In this, as in the American species, the head is very broad, and the forehead arched; but the horns are longer, more curved, and end in a finer point than those of the American Bison. The eyes are large and dark; the hair on the forehead is long and wavy; under the chin and on the breast it forms a sort of beard. In winter, the whole of the neck, hump, and shoulders are covered with a long woolly hair of a dusky brown colour, intermingled with a short soft fur of a fawn colour. The long hair is gradually cast in the summer, to be again renewed as the inclemency of winter comes on. The legs, back, and posterior portions are covered with short, dark brown hair. The tail is of a moderate length, is covered with hair, and terminates in a large tuft. The females are not so large as the males, neither are they characterised by that abundance of hair on the anterior parts, which is so conspicuous in the bulls.