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Show 694 DR. E. LONNBERG ON THE [June 19, researches, and material of that kind was not on hand, but indications of a development in such a direction as described above are not wanting. In a section through tbe core at this stage, it is plain that the bony tissue on the outer side is more spongy and has a more rugged surface than on the median side, where the tissue is very dense and has a smoother surface. The result of this is that the direction of the born-core becomes still more vertical, as is shown in figs. 1 C (p. 688) and 3 (p. 691), and the whole horn is broken downwards. This is effected with such force that the thin basal wall of the lower side of the horny sheath, which is too weak to withstand tbe pressure exercised by the growth of the upper parts, is bent angularly, as can be seen in fig. 2 (p. 689) and in fig. 4 (p. 692). When this last change has taken place the horn may be regarded as fully developed. The shape of the horns as well as their direction is somewhat different in different individuals. In some they are directed more forwards than in others'. The horns are often not even symmetrical in the same animal. Especially are the bases unlike, one being higher or broader than the other and so forth. Tbe most remarkable feature in the formation of the horn of the Musk-ox is perhaps the great changes to which the bony horn-core is subjected. Through reabsorption and apposition, it changes its shape and direction again aud again. In the development of the horn of the G n u an analogy to this is found. The first core of the young horns, which grow straight as spikes (cf. Blaauw, I. c), forms nearly a right angle to the future core of a later period, and must therefore be completely reabsorbed to allow the growth of the horn. But also in the twice-bent horn of the Hartebeest no growth of the horn is possible without reabsorption and apposition, because the horny sheath cannot glide upward on the core-the less so as the middle portion of the sheath is narrower than the distal and proximal portion2. But straight, aud many spiral, horns can easily grow without reabsorption of the horn-core, because the sheath can glide upwards without difficulty as its length is increased by basal growth. Sect. 2.-DESCRIPTION OF THE HOOFS OF THE MUSK-OX. The hoofs of the Musk-ox have a very characteristic shape. The two main hoofs of each foot display on the whole a broadly rounded shape with posteriorly heart-shaped outline. The transverse diameter of the lower surface of the foot is as long posteriorly as at the middle, and a few millimetres longer than the longitudinal diameter. Each hoof has a convex upper and outer surface, is rounded anteriorly, has a deeply concave lower, and likewise very 1 The animal which was the original of fig. 1 C (p. 688) had, for instance, its horns directed more laterally than in some other specimens. 2 Some years ago Professor T. Tullberg drew the attention to this fact at a meeting of the Zoological Section of the Natural History Society of Upsala. |