of the head, and increase in the freedom of movement
of the shoulder girdle and forelimbs (Olson, 1961:214). As Olson has
pointed out, this further localized the stresses to which the bone was
subjected. Additional localization of stresses was created with the
origin and development of tetrapods (reptiles) that were independent of
an aquatic environment and were subjected to greater effects of gravity
and loss of bouyancy in the migration from the aqueous environment to
the environment of air. The localization of these stresses was in the
border area of the cheek, away from its center.

What evidence is available to support this analysis of hypothetical
forces transmitted through the fully-roofed skull of such an animal as
_Captorhinus_?

It is axiomatic that bones or parts of bones that are subject to
increased stress become thicker, at least in part. This occurs
ontogenetically, and it occurs phylogenetically through selection. Weak
bones will not be selected for. Figure 10 illustrates the pattern of
the areas of the skull-roof in the temporal region that are marked on
the internal surface by broad, low thickened ridges. The position of
these ridges correlates well with the position of the oriented stresses
that were presumably applied to the skull of _Captorhinus_ during life.
It can be seen from Figure 10 that the central area of the cheek is
thinner than parts of the cheek that border the central area. The
thickened border areas were the regions of the cheek that were
subjected to greater stress than the thin central areas.

External evidence of stress may also be present. The pattern of
sculpturing of _Captorhinus_ is presented in Figure 11. The longer
ridges are arranged in a definite pattern. Their position and direction
correlates well with the thickened border of the cheek, the region in
which the stresses are distinctly oriented. For example, a ridge is
present on the internal surface of the squamosal along its dorsal
border. Externally, the sculptured ridges are long and roughly
parallel, both to each other and to the internal ridge.

The central area of the cheek is characterized by a reticulate pattern
of short ridges, without apparent orientation. The thinness of the bone
in this area indicates that stresses were less severe here. The random
pattern of the sculpture also indicates that the stresses passed in
many directions, parallel to the plane of the cheek and obliquely to
that plane.

_Possibl