118 ZOOLOGICAL GARDENS. 
furnish no better grounds of distinction ; and even the 
horns, which are the peculiar property of the order, 
are subject to so much variation in closely allied 
species, and pass by such easy gradations from the 
form that is considered typical of one group to that by 
which the neighbouring genera are characterized, as to 
lose much of their prima facie value for the purposes of 
classification. On these, however, we are in a manner 
compelled, by the absence of more efficient means ‘of 
discrimination, to rely as the basis of all our subdi- 
visions. 
The horns of the Antelopes are for the most part 
perfectly simple, one or two paradoxical species alone 
exhibiting a single short ramification. They are some- 
times common to both sexes; but more frequently exist 
only in the male. Internally they are formed by an 
osseous protuberance of the frontal bone, generally solid 
in its consistence like the deciduous horns of the Stags, 
and destitute of cavities or cells. By this latter cir- 
cumstance alone, combined with the roundness of their 
external contour, are the horns of some of the species 
to be distinguished from those of the Goats, which are 
always hollow within, containing pervious cavities com- 
municating with the frontal sinuses. There is reason, 
however, to believe that the latter structure lkewise 
obtains in several well marked Antelopes. The bony 
processes are permanently attached to the skull, and 
are enveloped on the outside by a sheath of horny 
matter, increasing by successive layers from the base in 
proportion to the growth of the nucleus within. This 
outer covering, to which the name of horn is more 
peculiarly appropriate, is rounded in its outline in all 
the genuine Antelopes, and is most commonly marked 
by circular elevated rings or by a continuous raised 
spiral line. In some few it 1s perfectly smooth through- 
