54 THE NATURAL HISTORY OF 



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 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 specially distinct ; 

 they were respectively 3 feet 8J in. and 3 feet 9^ 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, If inches to 2 inches apart, as 

 in the Simla morio of Professor Owen. The teeth, how- 

 ever, are immense, equalling or surpassing those of the 



