ii8 The American Naturalist. [March, 
above this articulation, forming a second process above the 
postzygapophysis, the episphen. This occurs in the Suoidea 
and the Booidea (Plate IV., figs. 2 and 3, Dicotyles and 
The mechanical explanation of the origin of this structure 
is probably found in the nature of the movement of that part 
of the vertebral column which is between the limbs during 
progression ; and especially of the more flexible region 
(lumbar) which is posterior to the ribs. All the gaits of 
quadrupeds may be reduced to three types, and their varieties. 
In the first, the extremities of the column are alternately 
elevated and depressed, without lateral motion. This is seen 
in the run- In the second, the sides of the column are alter- 
nately elevated and depressed. This is seen when the limbs 
of one side move simultaneously, as in the pace. In the third 
type, limbs of opposite sides of opposite extremities, move 
together, as in the walk and trot. The effect of this move- 
ment is to twist the column in its long axis. These effects 
are diagrammatically represented in the accompanying figure 
15. It is this torsion which has produced the involuted zyga- 
pophyses, and later the episphen. It should be the fact that 
animals which display this structure should walk and trot, 
while others should pace and run. And this is the case. The 
trot as a habitual gait is especially characteristic of the Di- 
plarthra. The Proboscidia and Carnivora pace, although the 
dogs frequently trot as well. We must suppose that the trot 
was the favorite gait of the Creodonta, since they possess 
the involuted zygapophyses. 
The only genus certainly referable to the DlCHODONTlD.4-:, 
is the Dichodon Owen, from the upper Eocene of England. 
In this form we have the earliest quadriselenodont molars, 
the intermediate fifth crescent having disappeared The first 
superior premolar is like a true molar, while the first inferior 
is trilobate (Kowalevsky ; molariform, Owen). The other 
premolars are very elongate and compressed, resembling those 
of Xiphodon. This resemblance is heightened by the incisi- 
form shape of the canines, and the uninterrupted dental series. 
In the same beds occur limb and foot bones which probably 
belong to Dichodon (Schlosser) which are didactyle, but in 
