Explanation of Plate. 



Figs. 12 and 13. — Have been designed to show how important an index of prog- 

 nathism is represented by the size of the spheno-maxillary angle. In 

 Fig. 12 are shown tracings from mesial sections of the skulls of the orang 

 (above) and the gorilla (below). In the former this angle (B Pr P) was 

 found to be 150", whereas in the gorilla it measured 123°, thus considerably 

 reducing the degree of prognathism. The anterior limb of the angle 

 (Pr P) gives one the impression of a pendulum which is gradually swinging 

 backwards. The dotted line inside the crania indicates the approximate 

 position of the tentorium cerebelli. Note that in the skull of the orang 

 the cranial base lies in one continuous horizontal plane. 



