t have been appressed. The depressed surface included the posterior
wing of the jugal, the whole of the squamosal, and probably the
anteriormost parts of the quadratojugal. Expansion of the muscle
rostrally was prevented by the postorbital strut that protected the
orbit (Romer and Price, 1940:53).

The sphenacodonts possess the primitive rhynchocephalian kind of
palate. In _Sphenodon_ the anterior pterygoid muscle arises from the
dorsal surface of the pterygoid bone and from the adjacent bones. A
similar origin suggests itself for the corresponding muscle, the second
major adductor mass, in _Dimetrodon_.

From the origin the muscle passed posterodorsad and laterad of the
pterygoid flange. Insertion was in the notch formed by the reflected
lamina of the angular, as suggested by Watson (1948).

In _Dimetrodon_ the relationship of the dorsal surface of the palate
and the ventromedial surface of the mandible in front of the
articulation with the quadrate is unlike that in _Captorhinus_. When
the mandible of _Dimetrodon_ is at rest (adducted), a line drawn
between these two areas is oblique, between 30 and 40 degrees from the
horizontal. Depression of the mandible increases this angle. The
insertion of the anterior pterygoid is thus always considerably below
the origin, permitting the muscle to be active throughout the movement
of the mandible, from maximum depression to complete adduction. This
was a major factor in adding substantially to the speed and power of
the bite.

The presence and extent of a posterior pterygoid is more difficult to
assess, because of the closeness of the glenoid cavity and the raised
ridge of the prearticular, and the occupancy of at least part of this
region by the anterior pterygoid. In some specimens of _Dimetrodon_ the
internal process of the articular is double (see Romer and Price,
1940:87, Fig. 16) indicating that there was a double insertion here.
Whether the double insertion implies the insertion of two separate
muscles is, of course, the problem. Division of the pterygoid into
anterior and posterior portions is the reptilian pattern (Adams, 1919),
and such is adhered to here, with the posterior pterygoid arising as a
thin sheet from the quadrate wing of the pterygoid and the quadrate,
and inserting by means of a tendon on the internal process of the
articular, next to the insertion of the anterior pterygoid.

[Illustration: FIG. 6. _Dimetrodon._ Internal aspect of right cheek,
show