nables
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 both
these kinds, according to Mr. Wallace, are devoid of excrescences, and
resemble the smaller males, but are shorter by 1 1/2 to 3 inches, and
their canine teeth are comparatively small, subtruncated and dilated
at the base, as in the so-called 'Simia morio', which is, in all
probability, the skull of a female of the same species as the
smaller males. Both males and females of this smaller species are
distinguishable, according to Mr. Wallace, by the comparatively large
size of the middle incisors of the upper jaw.

So far as I am aware, no one has attempted to dispute the accuracy of
the st