46 Proceedings of the Royal Physical Society. 



of the cranium. It is often absent; and when present 

 varies much in development in different animals, and even 

 in the same animal at different periods of life. 



£asion = th.e mid-point of the lower boundary of the 

 foramen magnum. 



Opisthion = the corresponding point on the upper border 

 of the same foramen. 



Further points will be mentioned in connection with the 

 diameters now to be described. 



Diameters. 



Length of Cranium. — It is impossible to satisfactorily 

 express the length of the cranium by the diameter employed 



Tronfo-pariefal leogfr). 



TVo rjto 



Fig. 4. 



by anthropologists, i.e., from the occipital point to the 

 glabella, because these do not exist in the bulk of mammals. 

 Nor would it be well to take the length of the cranium 

 as being expressed in a diameter measured from the pro- 

 jecting crest formed by the supra- occipital bone; for this 

 is very variable in the degree of its development in different 

 animals. Apparently the most satisfactory and definite 

 points between which a measurement may be taken are 

 the opisthion and the nasion (Fig. 4, d). It will be found 

 that this diameter bears a fairly constant relationship to the 

 actual cranial length in all mammals, excepting those, com- 

 paratively few in number, in which the occipital bone bulges 

 backwards beyond the level of the foramen magnum. 



Breadth of Cranium. — To express the breadth of the 

 cranium is more difficult than to indicate its length, because 



