0. C. Marsh—Notice of New Tertiary Mammals. 207 
changes in the posterior limb of these genera are very similar, 
but not so manifest, as the oldest type (Urohippus) had but 
ve toes in each foot, the typical number in mammals. This 
reduction in the number of toes may, perhaps, have been due 
to elevation of the region inhabited, which gradually led the 
animals to live on higher ground, instead of the soft lowlands 
where a polydactyl foot would be an advantage. 
he gradual elongation of the head and neck, which took 
place in the successive genera of this group during the Tertiary 
period, was a less fandamental change than that which resulted 
in the reduction of the limbs. The process may be said to have 
already began in Oroh:ppus, if we compare that form with other 
most nearly allied mammals. The diastema, or “place for the 
bit,” was well developed in both jaws even then, but increased 
materially in succeeding genera. The number of the teeth 
remained the same until the Pliocene, when the front lower 
very large in Orohippus, and in this genus, as well as those from 
the Middle Tertiary, appear to have been well developed in both 
orbit was not enclosed behind by an entire bridge of bone, an 
this first makes its appearance in this country in Pliocene 
Mg, likewise, in existing horses. 
the peculiarly equine features acquired by Ore _— 
Bondy throughout the entire series of succeeding forms. 
ch, e. gis the form of symphysial part of the lower jaw, 
