0. C. Marsh—Notice of New Tertiary Mammais. 255 
wears into an ear-shaped lobe, enclosing a pit filled with 
cement. The crowns of these molars are unusually short, even 
when unworn. They all have distinct fangs, and their enamel 
18 covered with cement. The outer lobes have only a faint 
indication of a median ridge on their concave faces, The 
buttresses that enclose these faces are prominent. 
Measurements. 
- Antero-posterior diameter of first upper true molar, - .__-- i 
Transverse diameter, ______. - . PEL, . +, 
Antero-posterior diameter of last upper MOT, ©... o <- 17° 
aAranaverse diameter, 2. ee 2155 
The specimens here described indicate that the genera An- 
chippus and Hypohippus, established by Dr. Leidy, are nearly 
related, and will probably prove to be identical. 
The large number of equine mammals now known from the 
Tertiary deposits of this country, and their regular distribution 
through the subdivisions of this formation, afford a good 
Opportunity to ascertain the probable lineal descent of the 
modern horse. The American representative of the latter is 
the extinct Equus fraternus Leidy, a species almost, if not en- 
tirely, identical with the old world us caballus Linn., to 
Which our recent horse belongs. Huxley has traced success- 
fully the later genealogy of the horse through European extinct 
forms,* but ; : be 
n 
Pus, and Pliohippus, of the Pliocene; and Equus, Quaternary 
and recent, 
he most marked changes undergone by the successive 
equine genera are as follows: Ist, increase in size; 2nd, increase 
1D speed, through concentration of limb bones; 3d, elongation 
of head and neck, and modifications of skul e increase 
In size is remarkable. The Eocene Orohi was about the 
size of a fox. Miohippus and Anchither'um, from the Miocene, 
Were about as large as a sheep. //ppar‘on and Pliohippus, of 
the Pliovene, equalled the ass in height: while the size of the 
Quaternary Equus was fully up to that of the modern horse. 
* Anniversary Address, Geological Society of London, 1870. 
