352 LAND MAMMALS IN TpE WESTERN HEMISPHERE 



kinds of modification diverged from a common starting-point 

 such as may be seen in the middle Eocene perissodactyls 

 generally, and that in each series the transformation was 

 gradual. 



(5) The modification of the skull followed several different 

 courses, as designated by the major and minor subdivisions 

 of families, subfamilies and phyla. The development of horns, 

 whether single or double, in transverse or longitudinal pairs, 

 was the most important single influence in transforming the 

 skull, as determined by the mechanical adjustment necessary 

 to make these weapons effective, but even in the hornless 

 forms changes went on, and in all the phyla the skxill departed 

 more and more widely from the primitive Eocene type in 

 each succeeding geological stage. The most aberrant form 

 of skull was that of the hornless and presumably aquatic 

 fMetamynodon, in which the greatly shortened face, high 

 sagittal crest and extremely wide zygomatic arches were alto- 

 gether exceptional. 



(6) When the history of any horned phylum is at all com- 

 plete, the development of the horns may be followed step by 

 step from the marks which they left upon the skull. As a rule, 

 the story was one of gradual enlargement, but, in one case at 

 least, an incipient horn apparently failed to enlarge and was 

 eventually lost. 



(7) In the light, slender and cursorial thj^acodonts the 

 mode of development resembled that of the horses, as appears 

 in the elongation of the neck, limbs and feet, in the enlarge- 

 ment of the median toe and concomitant reduction of the 

 lateral digits. Also, as in the horses, the elongation of the 

 limbs began to be noteworthy while the body-weight was small 

 and was consequently accompanied by great slenderness; as 

 the body-weight increased, the limbs became stouter, to yield 

 the necessary support. 



(8) In the phyla composed of massive animals the principle 

 of change agreed with that exemplified by the ftitanotheres, 



