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Show 1903.J ON THE LIMBS OF THE EQUIDCE. 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, 110 ungulate was ever wholly plantigrade in both feet; the nearest approach to this condition obtaining in the Lower Eocene Coi'yphodon, 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 improbable, 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 structures. 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 Cervidce will be aware that a certain number of representatives of that family-notably the reindeer, the White-tailed Deer, the Mule-Deer, and, in a rudimentary condition, the Elk,-are furnished on the inner side of the hock wTith 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 occurrence 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 Equidce, 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 the 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 |