I am in agreement with Watson's reconstruction of the origins for both the anterior and posterior pterygoid muscles. On a functional basis, however, I would modify slightly Watson's placement of the insertions of these muscles. Watson believed that the jaw of _Dimetrodon_ was capable of anteroposterior sliding. The articular surfaces of the jaws of _Dimetrodon_ that I have examined indicate that this capability, if present at all, was surely of a very limited degree, and in no way comparable to that of _Captorhinus_. The dentition of _Dimetrodon_ further substantiates the movement of the jaw in a simple up and down direction. The teeth of _Dimetrodon_ are clearly stabbing devices; they are not modified at all for grinding and the correlative freedom of movement of the jaw that that function requires in an animal such as _Edaphosaurus_. Nor are they modified to parallel the teeth of _Captorhinus_. The latter's diet is less certain, but presumably it was insectivorous (Romer, 1928). With the requisite difference in levels of origin and insertion of the anterior pterygoid in _Dimetrodon_ insuring the application of force throughout the adduction of the jaws, it would seem that the whole of the insertion should be shifted downward and outward in the notch. If this change were made in the reconstruction, the anterior pterygoid would have to be thought of as having arisen by a tendon from the ridge that Watson has pictured (1948:828, Fig. 3) as separating his origins for anterior and posterior pterygoids. The posterior pterygoid, in turn, arose by tendons from the adjoining lateral ridge and from the pterygoid process of Romer and Price. Tendinous origins are indicated by the limitations of space in this area, by the strength of the ridges pictured and reported by Watson, and by the massiveness of the pterygoid process of Romer and Price. Discussion A comparison of the general pattern of the adductor musculature of _Captorhinus_ and _Dimetrodon_ reveals an expected similarity. The evidence indicates that the lateral and medial temporal masses were present in both genera. The anterior pterygoid aided in initiating adduction in _Captorhinus_, whereas in _Dimetrodon_ this muscle was adductive throughout the swing of the jaw. Evidence for the presence and extent of a pseudotemporal muscle in both _Captorhinus_ and _Dimetrodon_ is lacking. The posterior division of the pterygoid is small in _Captorhinus_. In _Dimetrodon_ this muscle has been reconstructed by Watson as a major adductor, an arrangement that is adhered to here with but slight modification. The dentition of _Captorhinus_ suggests that the jaw movement in feeding was more complex than the simple depression and adduction that was probably characteristic of _Dimetrodon_ and supports the osteological evidence for a relatively complex adductor mechanism.