1 889.] The Artiodactyla. 117 
this articulation, and it contains but one sesamoid bone, to 
which the trochlear surface is moulded in a concave surface, 
as in the case of the patella and the rotular groove of the 
femur (figs. 8, 9B, 9, 10, 14). 
The increased complexity oi the intervertebral articula- 
tions,' is seen in the modifications in the shapes of the zyga- 
pophyses.' In reptiles the mutual articulating surfaces of these 
processes are horizontal and flat. In the lower Mammalia 
they are slightly oblique. In many Carnivora the obliquity 
is strongly marked, and a similar form is seen in the lower 
-fE... 
- £-^ ,=:^ 
Diplarthra. As we ascend the scale of the latter, the pre- 
zygapophyses become involute and embrace the postzygapo- 
Physes above, as well as externally below (Plate IV., fig. i, 
Antilocapra). This superior part of the prezygapophyses 
develops, and reaches the basis of the neural spine, with which 
it forms an articulation. The base of the spine expands 
' For a tabular exhibit of these, see Proceedings Amer. Ass. Adv. Science, 1SS3 ; 
^'ngin of the Fittest, 1885. 
^ On torsion in locomotion. Seeart. Perissodactyla, Naturalist, 1S88. 988^ 'o73- 
