The cranial cavity is very small, the orbit confluent with the temporal fossa, and the zygomatic arch massive.
In Ornithorhynchus the zygomatic arch is much stouter than in Echidna.
As yet there is no clear evidence that reduction of the zygomatic arch is of adaptive value.
In sixteen cases the zygomatic processes were unequal and very prominent.
The skull has each of the zygomatic arches and the anterior end of the nasals broken through.
In addition the zygomatic ridge of the squamosal decreases in relative size with decrease in actual size of the skull.
Similarly the zygomatic breadth slightly exceeds one half of the basilar length, while in Sus cristatus it is less than half.
So extremely poor is the frontal development of this skull, that its diameter at the zygomatic processes is barely 3.5½.
The anterior (zygomatic) process of the squamosal is greatly reduced (only about one-third the length of the posterior process).
The braincase is nearly as broad as in the Bornean species, but the zygomatic width is distinctly less.
zygomatic zy·go·mat·ic (zī'gə-māt'ĭk, zĭg'ə-)
Of, relating to, or located in the area of the zygoma.