MESOHIPPUS. 



45 



OREODON MESOHIPPUS ZONE. 2. MIDDLE OLIGOCENE. 



As DESCRIBED ABOVE, THIS ZONE IS DIVIDED INTO THREE LEVELS, VIZ., LOWER OliEODON ZONE, MIDDLE OrEODON 



Zone ( = Metamynodon Zone), Upper Oreodon Zone, each of which contains distinct specific stages 



OR MUTATIONS. All THE SPECIES OF HORSES KNOWN IN THESE OVERLYING HORIZONS EXHIBIT A HYPOSTYLE 

 ON THE SUPERIOR MOLARS. THE HORSES ARE BETWEEN EIGHTEEN AND TWENTY INCHES IN HEIGHT AT THE 

 WITHERS. 



Species Attributed to Lower Oreodon Zone 2. 



Mesohippus bairdii Leidy, 1851. 



Plates 1.3,4, 2.4,13, 39.15,18, 51.7,11. Text Figs. 25, 26. 



Palceotherium bairdii, Lcidy, Joseph. [Communication without title] Proc, Acad. Nat. Sci. Phila., Vol. V, No. VI, (1850), Jan. 

 1851, p. 122. (Read Dec. 17, 1850.) No figure. 



Anchitherium bairdii Leidy, full description and first figure, Leidy, Joseph. ''Ancient Fauna of Nebraska: or a Description of 

 Remains of Extinct Mammalia and Chelonia, from the Mauvaises Terres of Nebraska," Smiths. Contr. to Knowl., Vol. VI, 1853, PI. 

 x, figs. 20, 21, PI. xi, figs. 1, 8. 



Fig. 25. Original figures of type and paratype of Mesohippus bairdii Leidy. (Upper and lower right figures) Palatal 

 and side views of type skull, after Leidy, 1853, respectively PI. x, fig. 21, PI. xi, fig. 1 ; palatal view natural size, side view 

 onedialf natural size. (Left lower) Side view of paratype lower jaw, one-half natural size, reversed in reproduction from 

 Leidy figure. After Leidy, 1853, PI. xi, fig. 5. 



Horizon and Locality. — Probably, from the head of Bear Creek, Pennington County, South Dakota, the locality 

 i^S^Oc^^— (fv^&lL 

 visited by Thaddeus Culbertson, the collector of this type. 



Type and cotypc. — (Leidy) " Paheotherium. — . . . . A second species was founded upon the cranium and a portion of 



the face containing the true molars; and the six superior and inferior molars of both sides of another individual." (Proc. 



