694 dk. e. lonxbeeg 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 the 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. 68S) 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 the 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 chauge 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. 



The 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 and again. In the development of the 

 horn of the Gnu an analogy to this is found. The first core of 

 the young horns, which grow straight as spikes (cf. Blaauw, /. 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 portion 2 . But straight, and 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 outliue. The trans- 

 verse 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. 



