projecting more than an inch from the head. The very late appearance of the ossicones 

 in the Okapi is a generic character, and possibly applies to the female as well as the male. 

 At present we do not really know any characters indicating the sex of skins or skulls of 

 Okapi. Of the series of sknlls measured by me, five are markedly broader than the rest, 

 giving, as explained in the diagrams, the indices 198'9 (C, Ward's B. M.), 1977 (J, 

 Johnston's smaller), 1904 (O, Genoa), 189 (A, Johnston's larger), 189 (E, Powell-Cotton 

 private collection). The rest range from 170 to 150. It seemed to me at one time that 

 possibly a difference of sex might go with the difference in breadth of skull, but I have 

 not been able to bring any evidence in favour of that hypothesis, and, moreover, though 

 the relation given in my measurements is a simple statement of fact, I am not satisfied 

 with it as a correct expression of the character of the skull which suggests the term 

 " broadness." Any measurement of the proportion of breadth to length in Okapi skulls 

 is complicated by the variation in the proportionate length of the premaxillae, the nasals, 

 the frontals, and the occipital region of the skull. Another character which differs in the 

 skulls studied by me is the form of the orbital margin, which in some cases tends to be 

 somewhat quadrangular, whilst in others it is more nearly circular. For illustrations of 

 this structure and of the praeorbital tubercle, which varies also in form and prominence, 

 reference should be made to Plates 8, 9, 10, 11 and 12. 



The asymmetry of the skull of Okapi, especially in regard to the size and the 

 direction of the axes of the ossicones, is noteworthy. Sometimes the right and sometimes 

 the left ossicone is the longer (compare Plate 2 with Plate 5). 



It appears that not only in the striping of the skin, but in regard to many osteo- 

 logical features such as breadth of skull, proportions of cranial bones, exact form of 

 sutures, size of tympanic bulla, size and skewness of ossicones, early or later development 

 of the latter, and as to many features in the proportions of the " apophyses " or processes 

 of the vertebras (see Plates 23 — 28), there is a very large range of variation in Okapis, no 

 two specimens being closely alike. The range of variation is indicated in the illustrative 

 figures contained in this atlas, and seems to me remarkable. At the same time I must 

 emphasise the fact that I have not been able to find any records of the presence or 

 absence of such variations in a series of specimens of any other wild mammal occurring 

 in a limited area such as that inhabited by the Okapi, and I am unable to say (without 

 further examination of series of tlie kind — not easily to be brought together — say, of a 

 species of Antelope, or of B<>8 or other such Pecorine genus) that Okapi is really excep- 

 tional in the amount of individual variation which specimens of it present. 



E. RAY LANKESTER. 



July, 1910. 



VI 



