of the total length of the
skull.

[Illustration: FIG. 7. _Thrinaxodon._ Showing masseter and temporal
muscles. Skull after Romer (1956). Approx. x 7/10.]

The coronoid process in _Thrinaxodon_ sweeps upward posterodorsally at
an angle oblique to the long axis of the ramus. Angular, surangular and
articular bones extend backward beneath and medial to the process. The
process extends above the most dorsal point of the zygomatic bar, as in
_Didelphis_. The mandibular ramus is ventrally convex in both genera.

The relationships described above suggest that _Thrinaxodon_ and the
therapsids having similar morphology in the posterior region of the
skull possessed a temporal adductor mass that was split into major
medial and lateral components (Fig. 7). The more lateral of these, the
masseter, arose from the inner surface and lower margin of the
zygomatic bar and inserted on the lateral surface of the coronoid
process.

The medial division or temporal arose from the sagittal crest and
supraoccipital crest and the intervening dermal roof. The muscle
inserted on the inner and outer surfaces of the coronoid process and
possibly on the bones beneath.

_Thrinaxodon_ represents an advance beyond _Dimetrodon_ in several
respects. The zygomatic bar in _Thrinaxodon_ extends relatively far
forward, is bowed outward and dorsally arched. Consequently, the
masseter was able to extend from an anterodorsal origin to a posterior
and ventral insertion. The curvature of the jaw transforms the
anterodorsal pull of the muscle into a dorsally directed adductive
movement regardless of the initial angle of the jaw. This is the
generalized mammalian condition.

With the development of the secondary palate the area previously
available for the origin of large anterior pterygoid muscles was
reduced. The development of the masseter extending posteroventrally
from an anterior origin presumably paralleled the reduction of the
anterior pterygoids. The therapsid masseter, as an external muscle
unhindered by the crowding of surrounding organs, was readily available
for the many modifications that have been achieved among the mammals.

In the course of synapsid evolution leading to mammals, the temporal
presumably became the main muscle mass acting in adduction of the lower
jaw. Its primacy is reflected in the phyletic expansion of the temporal
openings to permit greater freedom of the muscles during contraction.
In the synapsids that lead to mammals