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Show 1900.] STRUCTURE OF THE MUSK-OX. 687 Sect. 1.-THE DEVELOPMENT OF THE HORNS OF THE MUSK-OX. The peculiar shape, structure, and position of the horns of the Musk-ox make them more interesting objects for an investigation than the greater number of horns of other Cavicornia. The material on which this essay is based has been brought home by the Swedish Expedition to East Greenland in 1899, under the direction of Professor A. G. Natborst, and consists of a skull of a young calf and several skulls of adult bulls and cows. Unfortunately no intermediate stages are represented by young or half-grown animals, because no such animals were seen, Professor Nathorst informs me. This gap is to some degree filled by Sir John Bichardson's description and fine figure of the skull with the horn-cores of a yearling or "16 months old" bull '. With the aid of this description and figure, and above all by the extremely interesting markings of growth and structure which were made visible by preparing longitudinal sections of a horn of an old bull, I think that I shall be able to present a fairly exact sketch of the development of the horns of the Musk-ox. In preparing this I have had the valuable assistance of m y friend G. Svenander, Cand. Phil., who has made the accompanying drawings of horns in three different stages of development, and to whom I therefore beg to express inv best thanks. The origin of the horn-cores is conspicuous in the young summer calf as a slight prominence on the lateral surface of the frontal bone, about 1 cm. from the upper surface of that bone aud about as far from the fronto-parietal suture, but 3| cm. from the posterior margin of the orbit. The development of the horn must be slow during the first winter, because, as Mr. B. Lydekker informs me, the calves which were brought to Tromso by a Norwegian vessel, and subsequently sold to the Duke of Bedford, had not any horns even in December. In the second summer the growth must be rather rapid, as the horn-cores of a 16 months old bull according to Bichardson's figure (I. c.) measure about 11 or 12 cm. The author mentioned describes them in the following way :-" The horn-cores have a purely lateral origin, and do not rise at all above the facial line, but, springing from an almost cylindrical root immediately behind the orbits, stand out laterally with a moderate inclination basilad and antiniad, their axis forming with the mesial plane of the cranium an angle of 62°. These cores are moreover, in themselves, concave on their facial or coronal aspect, by which they receive a uniform upward curve in the direction of their length, in addition to their general direction of outwards, basilad, and forwards. Tbe tips of the cores in this yearling extend further from the sides of the skull laterally than any part of the massy core or its sheath in the four-year-old animal." With the help of this description, and assuming that the horns of an adult animal have not been 1 Zoology of the ' Herald,' p. 67, pi. iv. 45* |