42 



L. BOLK 



lamellae, on the top a system of tension lamellae will arise. In 

 the figure both systems are represented by some simple lines. I 

 admit it is a purely theoretical construction, which I have tried, 

 however, to bring in accordance with the principles of mechanics. 

 The point upon which I will lay some stress, is that the tension 

 lamellae necessarily must cross the median plane. And because 

 an interruption in their course by a suture would be contrary to 

 their mechanical function, the two frontal bones unite together. 

 Now we will examine in how far the anatomical conditions in the 

 different Primates agree with the principles worked out above. 



It is needless to give a long description of the anatomical 

 conditions in several specimens, for the inspection of some few 



crania suggests the regularity in the special groups of the Pri- 

 mates. I will confine myself therefore to treat each group as 

 a whole. 



The examination of the prosimian skull shows that in this 

 lowest group of Primates the frontal suture is a constant element 

 in the system of sutures, disappearing nearly at the same time 

 as the other sutures. I regret to have at my disposal only a 

 small number of skulls of prosimiae. Hence it is impossible 

 for me to give a summary of the age at which the metopical 

 suture disappears in the different genera of this group of Primates. 

 The small number of skulls in my possession indicate that a 

 considerable variability exists as to this point in the different 

 genera of the Prosimiae. So I found among five adult skulls of 

 Lemur only one specimen with the system of sutures still wholly 



