200 MR. R, LYDEKKER ON THE CALLOSITIES [ Mar. 3, 
There has, however, long existed an idea that the equine callo- 
sities are remnants of a vanished toe. Somewhat analogous to 
this idea is a theory, which has been advanced in a paper lately 
read before. the Royal Society of Edinburgh by Prof. Ewart’. 
That gentleman, it appears, has based his investigations to a 
very great extent on the condition obtaining in the fetus; and 
has arrived at the conclusion that the callosities in question have 
nothing to do with glands, but that they represent certain foot- 
pads of polydactyle mammals. According to Prof. Kwart’s view, 
the fore-callosity of the Horse is homologous with the supplemental 
pad on the fore-foot of the Dog, while the hind callosity (which is 
wanting in all existing Hquidw save the true Horse and the so- 
called Hguwus przewalskii) corresponds to the hinder plantar pad 
of the Banded Anteater (A/yrmecobius fasciatus). Such correlations 
will, I venture to think, scarcely be taken seriously by the great 
majority of zoologists ; and I shall therefore confine my attention’ 
to the question whether the identification of these callosities with 
foot-pads generally is well founded. In all cases my observations 
are confined to the condition obtaining in adult animals. It may 
be added that I purposely brought these observations to the 
notice of the Society before the publication of Prof. Ewart’s paper, 
since I treat the subject from a different standpoint. 
In the first place, the callosities on both limbs of the Horse are 
situated on the inner surface, whereas, if they represented 
vestigial foot-pads, their position should be, primd facie, on the 
hinder. aspect, as is the case with the ergot. It might, indeed, 
be argued that they have changed their original. position, but 
of such a shifting there is no evidence in the adult. A second, 
and perhaps more important, objection to the foot-pad theory may 
be drawn from the fact that the callosities in the fore-limb are 
situated above the so-called knee-joint (carpus), and are therefore 
altogether higher up than any of the foot-pads of plantigrade 
mammals. Unless, therefore, another shift of position has taken 
place, the fore-callosities do not represent foot-pads. This argu- 
ment, it may be mentioned, was used by Sir W. H. Flower to 
disprove the theory that the callosities are remnants of the lateral 
digits. 
The hind-callosities, on the contrary, are situated a short distance 
below the joint of the hock (tarsus), and are therefore on a part 
of the limb, albeit on its inner side, which is included in the foot 
of a plantigrade mammal. If, however, the front callosity be 
regarded as serially homologous in a general sense with the hind 
one—and this is an integral part of Prof. Ewart’s theory,—it will 
be evident that in the event of the former not being a foot-pad, 
the same will hold good for the latter. 
A third, and perhaps stronger, objection may be urged against 
the foot-pad theory. On the assumption that the callosities of the 
existing Hquide are vestiges of foot-pads, it is clear that these 
1 See ‘ Nature,’ vol. lxvii. p. 239 (1908). 
