292 DR. E. EAY LANKESTEll ON OKAPIA. 



ajiplics equally to the substance " keratin," of which many horns larpiely consist, and 

 to the antler of the Stag, whicli has no horn about it. The " horn " of the Rhinoceros 

 consists of a solid fibrous growth of keratin, resting on a slightly raised bony boss. 

 There is no "horn" of this structure known among the Pecora. There are living 

 Pecora and there are allied living and extinct forms devoid of all trace of those growths 

 on tlie head which are called horns, whether of this or of that variety. Horns appear, 

 in proportion to their size and dominance, to be structures easily acquired and easily 

 lost, for they are often absent in the female whilst present in the male. They are 

 absent in both sexes in some domesticated races and present in others of the same 

 species; and, further, whilst commonly only a single pair are developed in Pecora, yet 

 a second pair may occur in a genus {Tetraceros) otherwise differing little from genera 

 with a single pair, and in domesticated races (Sheep) we have well-marked strains 

 without horns, with one pair, with two pairs, and even three pairs, and with large 

 and striking differences of shape and size. 



The horns of the Pecora are essentially bony outgrowths, or " ossicusps," developing 

 on and from the roofing-bones of the skull. I definitely adopt this theory of their 

 nature and origin, and reject that which Avould regard them as originating ancestrally 

 either as horny plates or knobs of the epiderm, or as detached subepidermal bony 

 plates like the carapace-elements of Glyytodon and Armadillo. The fact that in 

 Bovidge the bony horn-core or ossicusp makes its appearance as a detached plate 

 of tissue which ossifies from an independent centre, and only fuses with the subjacent 

 bone during ossification, does not necessitate the conclusion that this was the 

 ancestral mode of origin of the horn. 



The definite growth and selection of a pair or more " ossicusps," or exostoses, so as 

 to reach a size deserving the name of " horns," was probably preceded by a condition 

 of the skull in which " tumescence," such as we see in Okapia and Giraffa, was a 

 general tendency. Eounded bosses were formed here or there, both paired and median. 

 They gave definite point to a blow when the head was used as a battering-ram, and 

 also by their sinus-structure made fracture a matter of small consequence. Variation 

 and selection would emphasize one or other pair of such " bosses," or this or that 

 median boss, differently in different lines of descent. It is not necessary to suppose 

 that the paired horns of all Pecora are the lineal descendants of one and the same 

 pair of " bosses " in ancestral forms. It is obvious enough that one of the pairs of 

 horns of Tetraceros has an origin independent of that of the other pair, and so too 

 with the smaller and larger pairs of horns of Sivafherium and the median and posterior 

 pair of Bramatheriitm. The tendency in primitive Ungulates to develop nusnerous 

 bosses on the skull, which are incipient horns, is seen in the extinct group represented 

 by the genus JDinoceras, Avith its anterior, median, and posterior pairs of horn-bosses. 

 The paired horns of Giraffa are shown by the young specimen drawn in text-fig. 12 

 to be distinctly related to a pair of bosses on the parietal bones and not to correspond 



