232 ON THE NATURAL HISTORY 



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 versd. Now, those skulls which 

 have the largest and strongest jaws and the widest zygo- 

 matic 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 specifically 

 distinct ; they were respectively 3 feet 8 inches and 3 feet 

 9$ inches high, and possessed no sign of the cheek excres- 

 cences, but otherwise resembled the larger kinds. The 

 skull has no crest, but two bony ridges, If 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 i 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. 



