4 



osborn: OLIGOCENE, MIOCENE, PLIOCENE EQUID.E. 



All the above ratios and indices express adaptations to speed and weight respectively, as brought out in the recent in- 

 vestigations of Osborn and Gregory. There are many other speed and weight adaptations in the skeleton; the above 

 are the most frequently available and measurable. 



3. Digital and Phalangeal Reduction, and Degeneration. 



1. Tetradactyl, isotridactyl, anisotridactyl, monodactyl stages in the reduction of the lateral and acceleration of the 

 median digit, terminating in the monodactylism of Pliohippus and Equas. 



2. Stages in the reduction of the trapezium and of Mtc. V. 



3. Pollex. Digit I, absent in the known Equidse. No trace of Mtc. I has been found in any of the known Eocene 



parastyle mesostyle metaptyle 



Fig. 1. Chief elements of the equine molar teeth. 

 A (Upper) Crown of buno-lopho-selenodont upper molars in an Oligocene stage of evolution. 

 B (Lower) Crown of the lower molar in a Pliocene stage of evolution. 



After Osborn, 1907, fig. 160, p. 174. 



Equida? or, in fact, other Eocene Perissodactyla. Search by Matthew among the remains of Mesohippus, Parahippvs, 

 Merychippus, Pliohippus, and Hipparion has failed to confirm previous observations of the presence of even a vestige of 

 Mtc. L The bone which previously was mistaken for this vestige is the trapezium, a carpal which often descends on the 

 side of Mtc. II. 



4. Trapezium. Though losing its function of supporting Mtc. I, the trapezium (Plate 39) persists in the Oligocene 

 and Pliocene Equidre, and occurs as a variable element in the Pleistocene and modern Equus; it often descends on the side 

 of Mtc. II so as to imitate the vestige of Mtc. I. 



5. Displacement. Proximal expansion of Mts. III. Matthew has observed the following articulations: 



Mts. Ill articulates with ectocuneiform 

 only in all Lower and Middle Oligo- 

 cene species, so far as known. 



Mts. Ill articulates with ectocuneiform 

 and cuboid in all Upper Oligocene 

 species of Mesohippus and Miohippus; 

 also in Parahippus, Merychippus, Hip- 

 parion (all American species except 

 one), Protohippus. 



Mts. Ill articulates with ectocuneiform, 

 cuboid, and mesocuneiform, slightly 

 in Kalobatippus and lArchaohippus, 

 broadly in Anchitherium, Hypohippus, 

 Hipparion (all Old World species), 

 Pliohippus, Hippidium, Onohippidium, 

 Equus. 



