XVII] THE COMPARISON OF RELATED F0RM8 761 



appearance of the skull. But these differences in the conformation 

 of the skull, conspicuous as they are at first sight, will be found 

 easy to bring under the conception of a simple and homogeneous 

 transformation, such as would result from the application of some 

 not very complicated stress. For instance, the corresponding 



Fig. 394. Skull of Hymchyus agrariiis. (After Osbom.) 



Fig. 395. Skull of Aceratherium tridactylum. (After Osborn.) 



co-ordinates of Aceratherium tridactylum, as shown in Fig. 395, 

 indicate that the essential difference between this skull and the 

 former one may be summed up by saying that the long axis of the 

 skull of Aceratherium has undergone a slight double curvature, 

 while the upper parts of the skull have at the same time been 



