al 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.

In _Captorhinus_ the presence of an overlapping premaxillary beak
bearing teeth that are slanted posteriorly requires that the mandible
be drawn back in order to be depressed. Conversely, during closure, the
jaw must be pulled forward to complete full adduction. The
quadrate-articular joint is flat enough to permit such anteroposterior
sliding movements. The relationship of the origin and insertion of the
anterior pterygoid indicates that this muscle, ineffective in
maintaining adduction, may well have acted to pull the mandible
forward, in back of the premaxillary beak, in the last stages of
adduction. Abrasion of the sides of the inner maxillary and outer
dentary teeth indicates that tooth-to-tooth contact did occur. Whether
such abrasion was due to contact in simple vertical adduction or in
anteroposterior sliding is impossible to determine, but the evidence
considered above indicates the latter probability.

Similarities of _Protorothyris_ to sphenacodont pelycosaurs in the
shape of the skull and palate already commented upon by Watson (1954)
and Hotton (1961) suggest that the condition of the adductors in
_Dimetrodon_ is a retention of the primitive reptilian pattern, with
modifications mainly limited to an increase in size of the temporalis.
_Captorhinus_, however, seems to have departed rather radically from
the primitive pattern, developing specializations of the adductors that
are correlated with the flattening of the skull, the peculiar marginal
and anterior dentition, th