Pliohippus leidyanus Osborn, 

 Amer. Mus. 17224. (Left) p 3 of 

 type. 



Merychippus paniensis Cope, 

 Amer. Mus. 9460, p 3 , referred 

 specimen. 



Merychippus republicanus Osborn, 

 Amer. Mus. 8347, p 3 of the type. 



Merychippus fsphenodus Cope, 

 Amer. Mus. 8281, cotype. (Re- 

 versed in drawing). 



Hipparion coloradense Osborn, 

 Amer. Mus. 9094, type. (Reversed 

 in drawing). 



Merychippus calamarius Cope, 

 Amer. Mus. 14014, referred speci- 

 men. (Unworn, without cement.) 



Merychippus isonesus tertius Osborn, 

 Amer. Mus. 14180, m 2 of the type. 



Parahippus bremdens Marsh, 

 Yale Mus. 11274, type. 



ps 



is" es' 



hi" 



I" 



pi 



-Ay 



pi 



pi 



I" 



Parahippus texanus Leidy, 

 Amer. Mus. 12924, m 2 , referred >-> 

 specimen. 



Miohippus gemmarosw Osborn, 

 Amer. Mus. 13808, m 1 of the type. 

 (Reversed in drawing.) 



Mesohippus bairdii Leidy, 



Amer. Mus. 1477, m 2 of neotype. 



Mesohippus hypostylus Osborn, 

 Amer. Mus. 1180, m 1 of type. 



pi- 



er 



/>< me".~r>}* en" hi" 



ps? ep' hi 



hi" 



I 1 



s" me" ms" <:n" hi" 



Fig. 3. Evolution of upper and lower 



EQUINE MOLARS. 



(Left) Component cones, crescents, 

 crests, styles, crochets, and plicas of upper 

 molars in Oligocene to Pliocene stages of 

 evolution. 



(Right) Component elements forming 

 the six columns on the internal face of the 

 lower molars, namely: parastylid, meta- 

 conid, metastylid, entostylid, entoconid, 

 hypoconulid. 



ps parastylid es 

 me d metaconid en c 

 ms d metastylid hl d 



entostylid 

 entoconid 

 hypoconulid 



Pliohippus leidyanus Osborn, Amer. Mus. 

 17224, p4 of type, crown and outer view. 



Outer view. 



Merychippus calamarius Cope, 



Amer. Mus. 13901a, referred m2. (Re- 

 versed in drawing.) 



inner view. 



Merychippus isonesus primus Osborn, 

 Amer. Mus. 14187, mi of the type. 



Inner view. 



Parahippus, Amer. Mus. 14305, mi. 



Inner view. 



Miohippus gemmarosw Osborn, 



Amer. Mus. 12928, mi of paratype. 



Inner view. 



Mesohippus bairdii, referred specimen, 

 Amer. Mus. 12287, mi. 



Inner view. 



6 



