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Show 692 DR. E. L O N N B E R G O N T H E [June 19, of which the exostoses consist is reabsorbed and replaced by horny layers. It may be said metaphorically that the horny sheath eats down into the bony mass, which thus gets a rugged and pitted surface such as is seen on figs. 3 and 1 C. The pits and holes are filled up with horny substance. The interior surface of the basal part of tbe horny sheath is, in correspondence with this, densely beset with strong warty prominences, as can be seen in fig. 4. By this arrangement the horns become exceedingly strongly fixed to the skull, which is needed on account of the downward direction of the horn-core, which does not at all extend into the upward curve of the horn itself; every weight on or resistance against the hooked tip of the horn tends to pull the horny sheath of the core, and this would rather easily take place if it was not for this clinching of the Fig. 4. Lateral view of a longitudinal section of a horn of an old male Musk-ox, showing the structure of the inner part of the basal horn-layer, which has taken the place of the exostoses, and the lines indicating the periodical growth of the same. basal parts to the bony mass of the skull. The horn cannot consequently be regarded as ready for use as a weapon before it is fixed to the skull in the manner described. Such an arrangement is also much more suitable than if the core should extend into the upward curve of the horn, because, as it is, the horn at the bend can be entirely filled up with horny substance, and thus be made much stronger at this place, on which the heaviness of a charge on the tip of tbe horn works most. If the core extended through the curve for the purpose of fixing the sheath, the horn at this place should consist of a comparatively thin layer of horny substance which could be relatively easily bent, aud a central 'bone which would be broken even by a moderate charge. That it is of great importance that the horn should be strong at the curve, and that it is used for heavy work, is proved by the fact that one of the bulls shot by Professor Nathorst had tbe tip of one of its horns broken in the bend at a place where it was 4^ cm thick. Such a breaking needs certainly a formidable strength and a heavy load. The reabsorbing of the great exostoses and their replacement by |