1907.] HORNS OF THE GIRAFFE. 113 



ancestral form might in a series of generations shift its 

 position from the frontal to the parietal area ; and it might 

 reasonably be admitted that the upgrowth or tumescence 

 of the frontal ceased to develop when the parietal position 

 was assumed by the ossicone with consequent tumescence 

 and upgrowth of the parietal bone. 



5. On the other hand, the theoretical assumption that the 



frontally-placed ossicone of Okapi and the parietally- placed 

 ossicone of Giraffe are independent of one another and 

 possibly co-exist in an ancestral form, is favoured by the 

 fact that the Giraffe does develop a thiixl well-marked 

 ossicone in the mid-line of the frontal, and that both 

 Okapi and Giraffe exhibit minute supernumerary growths 

 of the kind on the cranial surface. 



6. It results from these considerations that it is not possible at 



present to trace the lateral horns of the Okapi and the 

 Giraffe into any close genetic relationship with those of 

 Bovidfe — still less of Cervidaj. At the same time it is 

 possible that the peculiar superficial element of the bony 

 horn (the ossicone) is identical in the lateral horns of 

 Okapi and Giraffa, having shifted its position backwards 

 in the latter genus. This conclusion is not, however, by 

 any means forced upon us since the Girafiid* are known 

 to have an additional ossicone — the median one ; and it is 

 therefore not without analogy that independent frontal 

 and parietal c sicones should develop. 



I am aware that i+ is not difficult to make assumptions by 

 means of which a genetic relationship between the lateral horns 

 of Giraffidse, Bovidse, and Cervidte is rendered possible ; but it 

 should, I think, be clearly understood that there is at present no 

 direct evidence to support these assumptions. It may be assumed 

 (a) that a bony horn of the nature of the horn-core of the Bovidas, 

 or of the antler-column of Cervidse, has in some remote ancestors 

 of the Giraffidse become segregated from the frontal bone of which 

 it was a part, and acquired independent existence as a fibrous 

 rudiment as well as indej)endent ossification, thus establishing the 

 independent lateral ossicone of the Giraffidse. Or, again, it may 

 be assumed (6) that in ancestors of the Bovidfe and Cervidse, 

 bony horns which were existing as free tegumentary products, 

 ankylosing in mature age with subjacent cranial bones, became 

 so ankylosed at earlier and earlier stages of development until all 

 trace of then- independent origin was lost, and they appeared to 

 originate as growths of the frontal bone itself. The stock so 

 endowed gave rise (it would be assumed) to Bovidse and Cervidse ; 

 that portion of the ancestry which retained the oiiginal method 

 of development of free tegumentary ossicones became, on the other 

 hand, the progenitors of the Giraffidse. 



I am not aware of any facts in the structure of living or extinct 

 Artiodactyla which furnish an analogy for either of these pro- 

 cesses of transformation. Nor do I think that our knowledge of the 

 extinct forms such as Scmiotheriuiii, Helladotherium, Sivcvtherium, 



Proc. Zool. Soc— 1907, No. YIII. 8 



