26 
PROCEEDINGS OF THE ACADEMY OF 
Merychtus medius, Leidy, 
The second species is founded upon a fragment of tlie lower jaw, containing 
the true molars, an isolated upper last true molar, and an isolated upper ca- 
nine tooth. * 
Length of series of lower true molars, 34 lines. 
Antero-posterior diameter of the upper last true molar, 14|- ' ' ' 
Merychyets major, Leidy. 
This species is proposed upon a fragment of the upper jaw containing the 
last pair of premolars and the succeeding pair of true molars. The latter teeth 
differ from the isolated upper molar of the preceding species in possessing a 
well defined basal ridge, of which the faintest traces only exist in M. elegans 
and M. medius. 
Length of space occupied by the two premolars and 
succeeding two true molars, 44 lines. 
Antero-posterior diameter of second true molar, 16 " 
SOLIPEDIA. 
ANCHrTHEKiuM (Hypohippus) affinis, Lcidy. 
A single specimen consisting of the crown of an upper molar tooth, in Dr. 
Hayden's collection, has the same form as the corresponding teeth of Anchithe- 
rium, except that the outer surfaces of its external lobes present no trace of 
median rising. It indicates an animal larger than A. aurelianense and about 
the size of Palceotherium crassum. 
Antero-posterior diameter externally 14 lines. 
Transverse diameter anteriorly 13^ " 
Anchitherium (Parahippus) cognatus, Leidy. 
The Niobrara collection contains three isolated unworn crowns of upper 
molar teeth, which have the same form as the upper deciduous molars of An- 
chitherium Bairdi or A. aurelianense, except that the outer extremity of the 
prolongation of the postero-internal lobe branches into several short folds. 
These latter have the same arrangement as similar but more numerous folds 
in the same position in Merychippus. 
Length of the series of three molars, 31 lines. 
Antero-posterior diameter of the first molar, 12 " 
Transverse diameter of the first molar 8 " 
Antero-posterior and transverse diameters of the 
third molar, 9 ' ' 
Independent of the remains of the anchitherioid genera Parahippus and 
Hypohippus, the collection made by Dr. Hayden contains numerous fragments 
of the skeleton of apparently six difi"erent equine animals, which, however, 
exhibit such an interchange of characters, that at present it appears im- 
possible to specify the isolated teeth and bones. The following genera and 
species appear to be indicated by the more characteristic specimens of the col- 
lection. 
Equus excelsus, Leidy. 
This species is about the size of the largest variety of the recent Horse, as 
indicated by molar teeth and bones of the limbs. The teeth do not differ in 
(;onstitution from those of the recent Horse ; and none of them present a greater 
degree of complication of the enamel folds on their triturating surface. 
Equus (Protohippus) perditus, Leidy. 
A second species of Horse, of small size, is especially indicated in the Nio- 
brara collection, by a fragment of an upper jaw containing the posterior four 
molars. The portion of jaw is like the corresponding part in the recent Horse, 
The molar teeth have their crowns about one-fourth worn. The enamel folds 
on the triturating surfaces are even less complex than in the recent Horse, and 
the antero-internal fold or column has the same form, direction and mode of 
[March, 
