ing anterior and posterior pterygoid muscles. Skull modified from
Romer and Price (1940). Approx. x 1/4.]

Watson (1948) has reconstructed the musculature of the jaw in
_Dimetrodon_ with results that are at variance with those of the
present study. Watson recognized two divisions, an inner temporal and
an outer masseteric, of the capitimandibularis, but has pictured them
(830: Fig. 4; 831: Fig. 5C) as both arising from the inner surface of
the skull roof above the temporal opening. But in _Captorhinus_ the
masseter arose from the lower part of the cheek close to the outer
surface of the coronoid process. Watson has shown (1948:860, Fig. 17B)
the same relationship of muscle to zygoma in _Kannemeyeria sp._ It is
this arrangement that is also characteristic of mammals and presumably
of _Thrinaxodon_. In view of the consistency of this pattern, I have
reconstructed the masseter as arising from the lower wall of the cheek
beneath the temporal opening.

Watson's reconstruction shows both the temporal and masseter muscles as
being limited anteroposteriorly to an extent only slightly greater than
the anteroposterior diameter of the temporal opening. The whole of the
posterior half of the adductor chamber is unoccupied. More probably
this area was filled by muscles. The impress on the inner surface of
the cheek is evident, and the extent of both the coronoid process and
Meckelian opening beneath the rear part of the chamber indicate that
muscles passed through this area.

Watson remarked (1948:829-830) that the Meckelian opening in
_Dimetrodon_ "is very narrow and the jaw cavity is very small. None the
less, it may have been occupied by the muscle or a ligament connected
to it. Such an insertion leaves unexplained the great dorsal production
of the dentary, surangular and coronoid. This may merely be a device to
provide great dorsal-ventral stiffness to the long jaw, but it is
possible and probable that some part of the temporal muscle was
inserted on the inner surface of the coronoid. Indeed a very
well-preserved jaw of _D. limbatus?_ (R. 105: Pl. I, Fig. 2) bears a
special depressed area on the outer surface of the extreme hinder end
of the dentary which differs in surface modelling from the rest of the
surface of the jaw, has a definite limit anteriorly, and may represent
a muscle insertion. The nature of these insertions suggests that the
muscle was already divided into two parts, an outer masseter and an
inner temporalis