zation of the dentition, the
shortening of the lacrimal, and the development of long vertebral
spines in _Dimetrodon_. The absence of gross differences in the areas
of the skull associated with the groups of muscles with which this
study is concerned, implies a similarity in the patterns of musculature
between the two groups. Romer and Price suggest that _Haptodus_,
although too late in time to be an actual ancestor, shows "all the
common features of the _Dimetrodon_ group on the one hand and the
therapsids on the other." The adductors of the jaw of _Dimetrodon_ were
probably little changed from those of the Haptodontinae and represent a
primitive condition within the suborder.

_Dimetrodon_ and _Captorhinus_ differ in the bones associated with the
adductor mechanism; the area behind the orbit in _Dimetrodon_ is
relatively shorter, reducing the comparative longitudinal extent of the
adductor chamber. Furthermore, the dermal roof above the adductor
chamber slopes gently downward from behind the orbit to its contact
with the occipital plate in _Dimetrodon_. Temporal fenestrae are, of
course, present in _Dimetrodon_.

_Musculature_

The adductor musculature of the lower jaw in _Dimetrodon_ was divided
into lateral and medial groups (Figs. 5, 6). The lateral division
consisted of temporal and masseter masses. The temporal arose from the
upper rim of the temporal opening, from the lateral wall of the skull
behind the postorbital strut, and from the dorsal roof of the skull.
The bones of origin included jugal, postorbital, postfrontal, parietal
and squamosal. This division may also have arisen from the fascia
covering the temporal opening (Romer and Price, 1940:53). The muscle
passed into the Meckelian fossa of the mandible and inserted on the
angular, surangular, prearticular, coronoid and dentary bones.
Insertion on the lips of the fossa also probably occurred.

The lateral division arose from the lower rim of the temporal opening
and from the bones beneath. Insertion was in the Meckelian fossa and
on the dorsal surface of the adjoining coronoid process.

[Illustration: FIG. 5. _Dimetrodon._ Internal aspect of skull, showing
masseter and temporal muscles. Skull modified from Romer and Price
(1940). Approx. x 1/4.]

The reconstruction of the progressively widening masseter as it
traveled to the mandible follows from the progressively widening
depression on the internal wall of the cheek against which the muscle
mus