Paleontology. 193 
structure in any of the existing hoofed animals, we 
find that the bony processes called zygapophyses, 
which belong to each of the constituent vertebra, 
are so arranged that the anterior pair belonging to 
each vertebra interlocks with the posterior pair be- 
longing to the next vertebra. In this way the whole 
series of vertebrze are connected together in the form 
of a chain, which, while admitting of considerable 
movement laterally, is everywhere guarded against 
dislocation. But if we examine the skeletons of any 
ungulates from the lower Eocene deposits, we find 
that in no case is there any such arrangement to 
secure interlocking. In all the hoofed mammals of 
this period the zygapophyses are flat. Now, from 
this flat condition to the present condition of full 
interlocking we obtain a complete series of connecting 
links. In the middle Miocene period we find a group 
of hoofed animals in which the articulation begins 
by a slight rounding of the previously flat surfaces: 
later on this rounding progressively increases, until 
eventually we get the complete interlocking of the 
present time. 
As regards teeth, and still confining attention to 
the hoofed mammals, we find that low down in the 
geological series the teeth present on their grinding 
surfaces only three simple tubercles. Later on a 
fourth tubercle is added, and later still there is de- 
veloped that complicated system of ridges and furrows 
which is characteristic of these teeth at the present 
time, and which was produced by manifold and 
various involutions of the three or four simple tuber- 
cles of Eocene and lower Miocene timcs. In other 
words, the principle of gradual improvement in the 
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