“Yet, when we examine the crania of these individuals, we find remarkable differences of form, proportion, and dimension, no two being exactly alike.  The slope of the profile, and the projection of the muzzle, together with the size of the cranium, offer differences as decided as those existing between the most strongly marked forms of the Caucasian and African crania in the human species.  The orbits vary in width and height, the cranial ridge is either single or double, either 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 separates 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 1 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 specially distinct; they were respectively 3 feet 81/2 inches and 3 feet 91/2 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, 13/4 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 11/2 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.