296 DR. E. RAY LANKESTER ON OKAPIA. 



bones in question are survivors of a general dermal cuirass, such as we see in 

 Armadillo, Ghjptodon, and Mylodon, or that they are new formations. The absence in 

 other Ungulata of dermal bones of either class renders it more probable tliat the 

 " separateness " of the horn-cores of Giraffa, and in a less degree of Bovidte, is an 

 acquired feature comparable to (if not identical in detail with) the " separateness " of 

 trochanteric and other epiphyses of bones in other parts of the skeleton. It is worth 

 noting in this connection that in skulls of old male Giraffes the periosteum at 

 various points — e.y., an additional median site and supraorbital sites — becomes active 

 and deposits rough growths of bones (exostoses), resembling those seen in old 

 rheumatic limb-bones of Man. 



The horns of Pecora, in reference to their complete structure and history, may be 

 grouped in four categories : — 



1. The Vellericorns of the living Giraffidte, with soft hairy integument covering the 



ossicusp : the ossicnsp either a cone-like upgrowth of the outer table of a 

 cranial bone with an accessory separately-ossifying " cap " or epiphysis or else a 

 simple exostosis (occipital horns of 5-horned Giraffe). 



2. The Cavicorns of Bovidse, with a horny theca : the ossicusp before ossification 



appearing as a knob of soft tissue separated from the subjacent cranial roof, 

 but subsequently fused to it. 



3. The Caducithecal C^y'icQxn^ oi Antilocapra, with a horny theca which is shed and 



replaced annually. 



4. The Antlers of Cervidae, direct outgrowths of the outer table of the frontal bones, 



clothed in an early stage with soft hairy integument, as in the vellericorns of 



Giraffidce, but losing this integument and exposing the distal portion of the 



bony ossicusp, which dies, is shed, and regrown annually, whilst the basal 



portion (Rosenstock) remains in perpetuity as part of the frontal bone. 



The cavicorns of the }3ovidge can be readily derived liypothctically from frontal 



vellericorns in such a position as the ossicusps of the Giraffid Samotherium (= Palceo- 



tragus). The development of a j)ermanent horny theca on the integument covering 



the ossicusp necessitates, if the horn so formed is to increase in size, that it shall grow 



only from the base : that is to say, it cannot develop branches which shall grow in 



proportion with the rest of the horn. Hence the permanent cavicorn is unbranched. 



The vellericorn covered by soft growing integument meets of course with no obstacle 

 in assuming a branching form in the course of growth, and perhaps the palmate horns 

 of Sivatherium were vellericorns. The form of the posterior horn-cores of Brama- 

 therium is, however, so much like that of a bovine horn-core that it seems probable 

 that these extinct Giraffidas had developed a horny theca upon the integument of their 

 horns. In that case the palmate horns oi Sivatherium not improbably had also a theca 

 which they must have shed from time to time during growth. 



