, there is no similar change in
the region of the palate that can be ascribed to the effect of the
pterygoid musculature, even though these adductors, like the temporal,
primitively were subjected to severe limitations of space.

Didelphis

Dissections reveal the following relationships of the external
adductors of the jaw in _Didelphis marsupialis_ (Fig. 8).

1. MASSETER

Origin: ventral surface of zygomatic arch.

Insertion: posteroventral and lateroventral surface of
mandible.

2. EXTERNAL TEMPORALIS Origin: sagittal crest; anteriorly
with internal temporalis from frontal bone; posteriorly with
internal temporalis from interparietal bone.

Insertion: lateral surface of coronoid process of mandible.

3. INTERNAL TEMPORALIS

Origin: sagittal crest and skull roof, including posterior
two-thirds of frontal bone, whole of parietal, and
dorsalmost portions of squamosal and alisphenoid.

Insertion: medial surface of coronoid process; dorsal edge
of coronoid process.

[Illustration: FIG. 8. _Didelphis marsupialis._ Showing masseter and
temporal muscles. Skull KU 3780, 1 mi. N Lawrence, Douglas Co., Kansas.
x 3/5.]

Temporal Openings

In discussions of the morphology and functions of the adductor
mechanism of the lower jaw, the problem of accounting for the
appearance of temporal openings in the skull is often encountered. Two
patterns of explanation have evolved. The first has been the attempt to
ascribe to the constant action of the same selective force the openings
from their inception in primitive members of a phyletic line to their
fullest expression in terminal members. According to this theory, for
example, the synapsid opening appeared _originally_ to allow freer
expansion of the adductor muscles of the jaw during contraction, and
continued selection for that character caused the openings to expand
until the ultimately derived therapsid or mammalian condition was
achieved.

The second course has been the attempt to explain the appearance of
temporal openings in whatever line in which they occurred by the action
of the same constant selective force. According to the reasoning of
this theory, temporal fenestration in all groups was due to the need
to decrease the total weight of the skull, and selection in all those
groups where temporal fenestration occurs was to further that end.

Both of these routes of inquiry are inadequa