PEEISSODACTYLA. 
261 
extent of these ridges or longitudinal expansion of the tubercles in the 
molars in both jaws, the oblique connections being still retained, gives the 
type of ’Equus. The elevation of the tubercles and deepening of the 
valleys give us the Selenodont tyjie of superior molars again in this genus; 
while the lower molars only differ from that type in having the crescents 
alternate instead of opposite, forming the Hippodont pattern. There can 
be little doubt that the line of the Horses comes through Hipposyiis (Hyra- 
cotlierium) from the Bunodonts, rather than through PalmtJierium, as sug¬ 
gested by some writers,” e. g., Huxley and others, 
I have not been able to ascertain the number of the digits from my 
specimens, but, according to Marsh, some of the species observed by him 
possess four on the anterior limb and three behind.* It has been shown by 
various authors that, as we extend our view backward in time, the number 
of digits in the series of Equine Perissodactyles increases; the modern one¬ 
toed Equiis and Pliocene Hippidium having been preceded by the Pliocene 
HippotJierium and Miocene AncJiitJieriiim, each with three toes. Hence, 
Marsh has supposed* that the four-toed Hyracotherium (OroJiippus) is the old¬ 
est ancestor of the line. This conclusion was published contemporaneously 
with my remarks quoted in the preceding paragraph, and is confirmatory of 
the position which I had taken the year previously. Finally, in Marsh’s 
article, it is observed that “ an earlier ancestor of this group, perhaps in 
the lowest Eocene, probably had four toes on this foot, and five in front. 
A still older ancestor, possibly in the Cretaceous, doubtless had five toes in 
each foot, the typical number in Mammals.” In my essay “ On the Ho¬ 
mologies and Origin of the Molar Teeth, etc.”, of the same date, f it is observed: 
“I trust that I have made it sufficiently obvious that the primitive genera 
of this division of Mammals [i. e., EcUtcabilia] must have been Bunodonts 
with pentadactyle plantigrade feet.” 
In a former essay, I alluded to the New Mexican species of this genus, 
under the name of Hipposyiis^X on account of the close resemblance between 
* Amer. Jonrn. Sci. and Arts, March, 1874. 
t March, 1874. 
I Eeport on Vertebrate Fossils discovered in New Mexico, 1874, p. 4 (U. S, Geog. 
Survs. W. of 100th M.). 
