Greenland Musk-Ox 323 



is strengthened by the fact that on the longitudinal section of the horn are 

 seen some lines of demarcation which are most easily interpreted as the 

 limits between such parts as have been added during different periods of 

 growth. . . . 



" During the second period ot growth (third summer) the upper side of 

 the horn is thickened by more rapid growth than that which takes place 

 on the under side. Through this a pressure is effected by the horny sheath 

 on the upper side of the horn-core, and this causes a reabsorption on the 

 upper side of the same. On the lower side, on the contrary, the pressure 

 is diminished, and therefore the horn-core is thickened below by apposi- 

 tion. In such a manner the direction of the main axis ot the horn is 

 lowered. At the same time the length of the horny sheath is increased 

 by basal growth, and it is driven out from the head in the direction of the 

 axis of the horn-core on which it grows. In connection with this, new 

 layers of horny substance fill up the end of the inner cavity ot the horny 

 sheath. 



" Next period (fourth summer) the growth is continued mostly in 

 the same way. The horny sheath is prolonged, and by stronger growth 

 on the upper side the main axis of the horn-core causes reabsorption on its 

 upper and apposition on its lower side. 



" In the following period the development continues also in the same 

 direction. The horn is lowered, the horn-core points more downwards. 

 At the same time that these changes are going on in the distal [terminal] 

 and middle portions of the horn, the base of the horn-core is enlarged and 

 expanded over a great part of the frontals and parietals, on which large 

 exostoses are developed. It is possible, although not fully proved, that the 

 prominences which can be seen on the skull of the summer calf a little 

 behind the first rudiments of the horn-cores, qjs the fronto-parietal suture, 

 have something to do with the formation of these exostoses. . . . 



" By-and-by the horny sheath encroaches in a median direction over 



