ON METOPISM 43 



intact, including the metopical suture. In the others the system 

 had completely disappeared. In three adult crania of Avahis 

 on the contrary, apparently of old individuals, all sutures includ- 

 ing the metopical, were still present, and so it was in two old 

 crania of Nycticebus. It thus seems that the sutures in Pro- 

 simiae close at a very different stage in the different genera of 

 this family. But for the present it suffices to know that the 

 disappearance of the metopical suture takes place simultaneously 

 with that of the other elements of the system. There is no spe- 

 cial factor necessitating the same to close at an earlier period 

 than the other sutures. In this respect the Prosimiae differ from 

 the monkeys and apes in which the closure of the frontal suture 

 always precedes those of the other sutures, and often very con- 

 siderably. From this we may conclude that the influence compel- 

 ling the metopical suture in monkeys and apes to disappear, is 

 absent in Prosimiae. Now a comparison of the topographical 

 relations between the temporal muscle and the frontal bone in 

 the lower and higher Primates, reveals that in Prosimiae the 

 muscle does not arise from the frontal bone at all. The reason 

 for this is obvious. In Prosimiae the lateral wall of the orbit 

 is a very incomplete one, and frequently also the floor of this 

 fossa is restricted to a foremost part. As a rule the outer wall 

 only is represented by an arch extending from the facial root of 

 the zygomatic arch to the parietal margin of the frontal bone. 

 This insertion of the orbital arch at the hindermost border of the 

 frontal bone causes the latter to be situated completely in front of 

 the temporal fossa, hence the temporal muscle cannot extend 

 its origin forward upon the frontal bone. In monkeys, as in 

 apes and man, the outer wall of the orbit is a complete one, 

 formed partly by the orbital surfaces of the zygomatic bone 

 and the great wing of the sphenoid bone. By this outer wall 

 the orbit is separated almost completely from the temporal fossa 

 and the plane of entrance of the orbit is considerably turned. 

 In Prosimiae the inclination of the latter is more a lateral than a 

 frontal one, the axis of the orbit making a more open angle with 

 the median plane. But in monkeys the plane of entrance is 

 turned, being directed principally forward and but slightly out- 



