1903. | ON THE LIMBS OF THE EQUIDA. 201 
structures must have existed in the ancestors of that family ever 
since the time when these ancestors were plantigrade. But, so far 
as I know, no ungulate was ever wholly plantigrade in both feet; the 
nearest approach to this condition obtaining in the Lower Eocene 
Coryphodon, in which the hind-limb was wholly plantigrade, while 
the front one was partially digitigrade. It has thus to be assumed, 
on the foot-pad hypothesis, that the front callosities of the Horse 
have been functionless structures from a period antedating the 
evolution of the Ungulata. Such a persistence, on exposed parts 
of the body, of a wholly functionless structure seems very im- 
probable, especially when the modifications are borne in mind 
which, on this hypothesis, the horse-line must have undergone 
since the time when the callosities were functional str uctures. 
Perhaps the case of the ergot may be cited against this argument ; 
but it should be remembered that this structure certainly acted 
as a functional pad at a much later stage of evolution than could 
possibly have been the case with the callosities. 
Having now stated what appear strong objections, so far as the 
adult is concerned, against correlating the callosities of the Horse 
with the foot-pads of polydactyle mammals, it remains to consider 
whether they can be identified with any other structures. Those 
familiar with the morphology of the Cervide will be aware that a 
certain number of representatives of that family—notably the rein- 
deer, the White-tailed Deer, the Mule- Deer, and, in a rudimentary 
condition, the Elk,—are furnished on the inner side of the hock 
with a glandular tuft corresponding very closely in situation with 
the hind-callosity of the Horse. In fact, the only difference in the 
position of the two structures is that the tarsal tuft of the Deer 
in question is placed rather lower on the hock. From the fact of 
its oceurrence in Deer so widely separated from one another as 
are the species mentioned, it seems evident that the tarsal gland 
(which is doubtless a scent-organ) is a very ancient structure, 
which was present in all the ancestors of the group, but has been 
lost, probably from disease, in the great majority of Old World 
forms. 
Judging from their position, there would seem to be a certain 
probability that the hind-callosities of the Horse and the tarsal 
gland of the Deer are homologous structures. 
With regard to the homology of the fore-callosity of the Zquide, 
it may be mentioned that many Gazelles have tufts of hair 
(“‘knee-brushes”) at the knee (carpus), which are probably 
glandular in origin. And it is possible (if the suggestion with 
regard to the hind-callosities hold good) that these may represent 
‘he fore-callosities of the Horse, for there seems no good reason 
why the position of a gland should not have somewhat shifted in 
two widely separated groups of mammals. Then, again, we have 
the carpal bristles of certain mammals, such as the Coatis and 
Dassies, already referred to as being regarded by Mr. Beddard as 
the remnants of a “scent-organ,’—a structure probably not far 
removed in its nature from a gland. The occurrence of these 
