much or little developed, and the zygomatic aperture varies considerably
in size. This variation in the proportions of the crania enables us
satisfactorily to explain the marked difference presented by the
single-crested and double-crested skulls, which have been thought to
prove the existence of two large species of Orang. The external surface
of the skull varies considerably in size, as do also the zygomatic
aperture and the temporal muscle; but they bear no necessary relation
to each other, a small muscle often existing with a large cranial
surface, and 'vice versa'. Now, those skulls which have the largest
and strongest jaws and the widest zygomatic aperture, have the muscles
so large that they meet on the crown of the skull, and deposit the bony
ridge which supports them, and which is the highest in that which has
the smallest cranial surface. In those which combine a large surface
with comparatively weak jaws, and small zygomatic aperture, the
muscles, on each side, do not extend to the crown, a space of from l to
2 inches remaining between them, and along their margins small ridges
are formed. Intermediate forms are found, in which the ridges meet
only in the hinder part of the skull. The form and size of the ridges
are therefore independent of age, being sometimes more strongly
developed in the less aged animal. Professor Temminck states that the
series of skulls in the Leyden Museum shows the same result."

Mr. Wallace observed two male adult Orangs (Mias Kassu of the Dyaks),
however, so very different from any of these that he concludes them to
be specifically distinct; they were respectively 3 feet 8 1/2 inches
and 3 feet 9 1/2 inches high, and possessed no sign of the cheek
excrescences, but otherwise resembled the larger kinds. The skull has
no crest, but two bony ridges, 1 3/4 inches to 2 inches apart, as in
the 'Simia morio' of Professor Owen. The teeth, however; are immense,
equalling or surpassing those of the other species. The females of