

South Dakota School of /Mines 97 



The adult animal averaged about eighteen inches in height, 

 approximately the height of the coyote. Mesohippus celer, a near 

 relative, found in the Lower Oligocene, was about two-thirds 

 this size, while Mesohippus intermedins, another near relative 

 of the Upper Oligocene, averaged approximately one-third 

 larger. Mesohippus bairdi was a slender-limbed creature very 

 well adapted for speed. The hind limbs were much longer than 

 the fore limbs, more so proportionately than in the present day 

 jhorse, and the spines of the lumber vertebrae were nearly if 

 not quite as high as those of the dorsal region, so that, accord- 

 ing to Farr, the rump must have been much elevated above the 

 withers if the different parts of the limbs were not very much 

 more flexed on each other than would seem justifiable, judging 

 from recent animals. Scott states that the obliquity of the faces 

 of the dorsal and lumbar vertebrae shoiv that the back was de- 

 cidedly arched. 



The skull was about seven inches in length (see Plate 31). 

 The brain was large and apparently well convoluted. It weighed 

 about one-third as much as the brain of the average present day 

 horse. The number of teeth was forty-four, the arrangement on 

 each side, above and below, as follows : Incisors, three ; canines, 

 one ; pre-molars, four ; molars, three. They were of the crested 

 or lophiodont type and show the intermediate stage in the con- 

 version of the short, round-knobbed enamel covered crown, into 

 the long, sharp-crested crown of cement, dentine and enamel, 

 as in the present day horse, so arranged that the unequal density 

 of these tissues produces a hard, uneven grinding surface at 

 all stages of wear. 



The most striking feature is the tridactyl nature of the 

 feet. There were three well-developed toes on each foot, fore 

 and hind. These represent the second, third and fourth toes of 

 five-toed animals. In addition to these, a splint bone on each 

 fore foot represents the fifth toe, and a small nodule of bone 

 is recognized as being the last lingering remnant of the first toe. 

 The middle or third toe is longer and larger than the lateral ones 

 and terminates in an enlarged, somewhat triangular bone, cor- 

 responding to the hoof bone of the present horse. Plate 33 

 shows in a striking way the general nature of the changes in the 

 obliteration of the lateral toes and the enlargement of the middle 

 one, the intermediate stage of which Mesohippus represents. 

 "As the hand is raised, we can understand why the thumb dis- 

 appears first, (Eohippus stage), because it was the first to leave 



