NATURAL SCIENCES OF PHILADELPHIA. 
27 
continuation as the postero-internal one. The space occupied hy the four teeth 
is about three inches in length, and ahout ten lines in breadth. Dr. Hayden's 
collection also contains bones of the limbs which correspond in relative size 
with the jaw and teeth above noticed. 
Figure 1, plate 7, of M. Gervais, Rech. s. 1. Mammiferes Fossiles de I'Amer- 
ique meridionale, apparently represents a species of the same equine subgenus 
as the above. No extent of attrition of the corresponding teeth of the domes- 
tic Horse will produce the same appearance indicated in this figure. 
Merychippus, Leidy. 
This genus is a much more remarkable one than could have been anticipated 
from an examination of the specimen alone from which it was first characterized. 
The specimen alluded to consists of the anterior upper two large molars, con- 
tained in a small fragment of the jaw, and was obtained by Dr. Hay den, at 
Bijou Hill. The two teeth are intermediate in form to the corresponding ones 
of Anchitherium, and the upper true molars of the Deer, and they bear a strong- 
resemblance to those referred above to the anchitherioid genus Parahippus. 
From the teeth of the latter, they differ only in having their intermediate lobes 
prolonged posteriorly past the inner conical lobes, so as to make them assume 
the appearance of the inner lobes of the upper true molars of ruminants. 
In a fragment of the upper jaw of a young animal, in the Niobrara collection, 
apparently belonging to a second species of Merychippus, the second and third 
temporary molars and their permanent successors are contained. The tem- 
porary molars have the same form as the teeth from which the genus was first 
characterized would have, in a more worn condition. They are invested with 
cementum, though in less quantity than is usual in the Horse, and it is more 
readily detached, which appears to have been the case in the two teeth from 
Bijou Hill. The crowns of the permanent teeth contained within the frag- 
ment of jaw under examination have the same form as the corresponding teeth 
of the recent Horse, with the modifications above noticed characterizing the 
subgenus Protohippus. 
Merychippus insignis, Leidy. 
Proc. A. N. S. viii. 311 ; 1857, 89. 
This species was characterized from the fragment of jaw containing what 
now appear to me to be the upper anterior two temporary molars, from Bijou 
Hill. The Niobrara collection contains a portion of the upper jaw, which 
appears to belong to this species, having an entire series of permanent molars, 
(exclusive of the small one,) so far worn as to be inserted by fangs. The series 
of teeth occupy a space of almost four inches in length and ten lines in breadth. 
Merychippus mirabilis, Leidy. 
Appears to be a distinct species from the former, of larger size. It is indicated 
in the Niobrara collection, by a specimen previously noticed, consisting of a 
fragment of the upper jaw, containing the second and third temporary molars 
and their permanent successors. Another specimen, in the same collection, 
belonging apparently to this species, consists of a fragment of the upper jaw 
of an adult individual, containing the back four molars, which are so far worn 
as to be inserted by fangs. The four teeth occupy a space of three and one- 
third inches in length and an inch in breadth. 
The two fragments of jaws, above mentioned, have a deep depression or 
lachrymal fossa in advance of the orbit, as in the Deer, Oreodon, &c. 
HiPPARION S. HiPPOTHERIUM OCCIDENTALE, Leidy. 
Hipparion occidentale : Pr. A. N. S. vii. 59 ; 1857, 89. 
The remains of this species were originally discovered by Dr. Hayden in a 
superficial deposit at White River, Nebraska. A number of molar teeth in the 
Niobrara collection appear to belong to the same species. 
HipPARioN s. HiPPOTHERIUM sPECiosuM, Leidy. 
Hipparion speciosum : Pr. A. N. S. viii. 311 ; 1857, 89. 
The remains of a smaller species of Hipparion than the preceding, were ori- 
1858.] 
