450 



H. F. Osbovn — Mammalia in North A 



merica. 



early seen in the lower molars, and at the same time both 

 upper and lower molars entirely sacrificed their primitive cut- 

 ting powers, and were converted from secodont into bunodont 

 types by bringing the primitive trigons down to the level of 

 the talons. At the same time, the upper molars acquired 

 intermediate tubercles, and the triangular or oblique arrange- 

 ment of the tubercles was shifted into the quadrangular or 

 transverse arrangement. This outline is the result of fifteen 

 years' observation. 



With square crowns {vs. triangular) and six conic cusps above 

 and below the molars of the Artiodactyl and Perissodactyl 

 Herbivora ended their first constructive period and started 

 upon their modernization. From this point we direct our 

 attention upon the numerous combinations of three or four 

 forms assumed by these single cones. The important thing 

 now is to determine at what period these combinations 

 were established, for there is wide difference of opinion as to 

 when ungulate divergence began. To this I refer later. 

 Taeker has recently shown how every modern embryonic 

 lophodont or selenodont molar first exhibits the archetypal 

 cones of the primitive bunodont. This law, together with my 

 own parallel studies of the evolution of the horse and rhinoce- 

 ros molars, led me to the discovery that these embryonic primi- 



A B 



The Limits of Variation. 



A, Merychippus. B, Aceratherium . Showing the secondary enamel foldings 

 of the crests arising from the centers of the ancestral cones. 



tive cones are also the main growth centers, for, in the upper 

 Miocene, long after the Perissodactyla have separated from 

 each other, we see the influence of the archetypal form in the 

 generic and specific variations of the molars. Compare the 

 teeth of Merychippus and of Aceratherium, and imagine that 

 you see underlying the diverse crests and .crescents the simple 

 bunodont molar of such a form as Hyracotherium leporinmn 

 of the London Clay. You will then notice that the charac- 

 teristic secondary folds and spurs of the Miocene teeth spring 

 from the old bunodont cones, that the two " cement lakes " of 

 Merychippus are equivalent to the two "fossettes" of Acera- 



