The supraoccipital crest flares upward to a considerable extent in _Thrinaxodon_ and slopes posteriorly from the skull-roof proper. The crest extends on either side downward to its confluence with the zygomatic bar. The area of the crest that is associated with the temporal musculature is similarly shaped in _Didelphis_. The zygomatic bar in each genus is stout, laterally compressed, and dorsally convex on both upper and lower margins. At the back of the orbit of _Thrinaxodon_, the postorbital process of the jugal extends posterodorsally. At this position in _Didelphis_, there is but a minor upward curvature of the margin of the bar. In _Thrinaxodon_ the dorsal and ventral postorbital processes, arising from the postorbital and jugal bones respectively, nearly meet but remain separate. The orbit is not completely walled off from the adductor chamber. The corresponding processes in _Didelphis_ are rudimentary so that the confluence of the orbit and the adductor chamber is complete. The adductor chamber dorsally occupies slightly less than half of the total length of the skull of _Thrinaxodon_; in _Didelphis_ the dorsal length of the chamber is approximately half of the total length of the skull. [Illustration: FIG. 7. _Thrinaxodon._ Showing masseter and temporal muscles. Skull after Romer (1956). Approx. × 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.