INTRODUCTION. 



7 



The evolution of these dental characters may be very readily followed in the comparative drawings of the grinding 

 teeth (Plate 3), also the profound changes of proportion observed at different planes from the summit to the base. The 

 transverse diameters remain nearly the same while the anteroposterior diameters rapidly decrease. 



Fig. 4. Equus niobrarensis Hay, referred specimen, Amer. Mus. 27S9. Left fourth superior premolar and first 

 molar, p^-m 1 . From the Hay^Springs Quarry, western Nebraska, Equus zone, Lower Pleistocene. Natural Size. 



The evolution of the lower molar teeth is correctly interpreted in Figs. 1,3. It may be readily traced from Mesohip- 

 pus into Pliohippus (Fig. 3) and thence into Equus (Fig. 4a). The six columns rise on the internal side of the crown from 

 before backward, as follows: 



Inner side 



ps d .... parastylid at the antero-internal angle 



me d .... metaconid metaconid, main column 



ms d . . . .metastylid " accessory column 



es entostylid rudimentary accessory column of entoconid 



en d entoconid main column of entoconid 



hl d hypoconulid " " " hypoconulid 



Outer side 



Ectostylid accessory external column between trigonid and talonid in cleft of outer wall. 



