ORDINAL TYPES OF MOLAKS : PKRISSODACTYLA 



177 



* , 



' 



m't'ms" "' 



Fie;. 1(J4. Evolution of the lower molars in the Equida. Morphological not phylogenetic. 

 (Cf. Fig. li \1.) 



A. Hi/i-'ii-nt/i' i-ium, Lower Eocene. 



B. Piichimolophus, Middle Eocene. Note appearance of metastylid (MI.V') by fissure of the 

 metaconid. 



D. M' Koliiiipus, Middle Oligocene. Note appearance of entostylid. (Marked msd). 



E. Hipparlon, Pliocene. Note great expansion and posterior fold of the metaconid 

 //<.<'), protostylid (ps f i), endostylid (i s'l). Worn tooth. 



F. Equus. All figures nitural size. Worn. 



FIG. 105. Premolar Terminnlogy, proposed by Scott and adoyited in this volume. Primitive Un- 

 gulate Types. Fourth upper premolar and first molar of A. Euprotoyoiiia, and B. Hyracotherium. 



Through all these higher stages the homologies with the primitive 

 tritubercular crowns can readily be traced. 



SPECIAL REFERENCES. 



Hatcher, J. B., "Recent and Fossil Tapirs," Amer. Jour. Sci. (4), Vol. L, 1896, 

 pp. 161-180. 



Wortman, J. L., and Earle, Charles, "Ancestors of the Tapir from the Lower 

 Miocene of Dakota," Bull. Amer. Mus. Nat. Hist., Vol. V., 1893, pp. 159-180. 



"Species of Hyracotherium and allied Perissodactjls from the Wahsatcli and 

 Wind River Beds of North America," Bull. Amer. Mus. Nat. Hist., Vol. VIII., 1896, 

 pp. 82-110, pi. II. 



Osborn. H. F., "The Extinct Rhinoceroses,'' Mem. Amer. J/x. Nut. Hist., Vol. I., 

 1898, pp. 75-164. 



Schlosser, M., " Beitrage znr Kenntniss der Stammesgeschichte der Hufthiere mid 

 Versuch einer Systematik der Paar- und Uiipaarhufer," Morphol, l<iln-l>. \~2. Especiallj r 

 pp. 97-112. 



M 



