442 The American Naturalist. (May, 
chenia, but the internal and external on each foot were very 
small, 
The Homalodontheride also had five toes on each foot, but 
with them all the toes are well-developed. The feet are strong 
and robust, and it is worthy of note that the hoof-bearing 
phalanges are split up, and have the same shape as those of 
the Chalicotherium of Europe. The carpal and tarsal bones 
have the alternate arrangement. The legs are equally robust, 
and the humerus hasan epitrochlear foramen. The bones of the 
feet and of the legs bear a singular resemblance to those of the 
Edentates, but there are also more characters of specialization 
altogether peculiar, without which they might be taken for 
the direct ancestors of the Edentates. The Homalodontheride 
are the ancestors of the more modern Chalicotheride of the 
Northern Hemisphere. It is a mistake to look -for the stock of 
the Chalicotheride in the Meniscotherid. The latter represent 
a type allied to the Proterotheridx, with which they ought to 
have a common ancestor as yet unknown. 
-~ The Proterotheride present us with peculiarities still more 
surprising. The discovery of the leg bones of the Thoatherium 
prove that it was a monodactyl like the horse. Moreover, in 
the Thoatherium minusculum, the reduction of the parts was still 
more advanced than in the horses; the posterior feet are very 
slender, provided with a single toe, the third, while the second 
and the fourth are represented ouly by vestiges of the meta- 
tarsals, much more atrophied than are the same bones in the 
horse. One sees, in this case, an example of parallel evolution 
very remarkable. A complete reduction of the bones of the 
foot among the ungulates has taken place in two different fam- 
ilies, at two different epochs, a fact perhaps without precedent: 
in the Vertebrate series, 
It is, however, certain that there exists a certain ancestral 
relation between the Proterotheridse and the Equide, for the 
latter have descended from a form approaching the Protere- 
therium, but with complete dentition. I have elsewhere stated 
that in Europe as in North America, one may be able to trace 
the genealogy of the horse beyond Anchitherium (Mesohippus ’ 
included), and that one ought to separate definitely from the: 
