Farr.] lo^ [May 15, 



(hypsodont), grow from persistent pulps and do not form distinct roots 

 until the animal is quite old, not until a length of crown is attained 

 Avhich under normal conditions will afford sufficient grinding surface for 

 an average lifetime. As the teeth wear oft' by attrition the loss is 

 replaced by growth, and growth and w'ear proceed pari-passu until the 

 animal becomes adult. 



The little Mesohippus, with its short-crowned (brachyodout) teeth, 

 inserted by distinct roots, must therefore have fed on succulent plants 

 that grew in swampy, marshy land — as if subjected to wear necessitated 

 by the mastication of the hard, silicious grasses of Miocene times, the 

 teeth would soon have worn out entirely and the animal would have 

 succumbed to starvation. In most of the specimens found the teeth are 

 only moderately abraided. 



The feet, too, being tridactyl are adapted to progression along the 

 oozy shore of rivers or to swampy, marshy ground as the toes would 

 spread and thus support the animal in the mud, while the monodactyl 

 foot of the horse is preeminently adapted for rapid locomotion over the 

 grassy plains. This would seem to prove that the life habits of the ani- 

 mal have changed very greatly during its evolution. Many of the 

 White River animals were adapted by their anatomical structure to life 

 in swamps. Some wei'e at least semi-aquatic in their habits, as is denoted 

 by the position of the posterior nares, which in some forms are removed 

 verj' far backward, e. g., Ancodus. 



The skull is equine in its characters, but is still quite small and the 

 facial region is short. The orbit is not enclosed behind. 



The neck is long, and, as in the horse, these vertebrae are larger than 

 those of the dorsal region of the column. The processes are not so mas- 

 sive as in Equus, but are quite as complex and are very well developed. 

 The spines of the dorsal vertebrae are not so high as we should expect, 

 and very evidently M. hairdi did not have any great elevation of the 

 anterior dorsal region. The modern horse is much higher at the withers 

 tlian at the haunches. The spines of the lumbar vertebrae are \qyj 

 high and incline forward at quite an angle. There is a very abrupt 

 transition in height of spines from the first sacral, which has a very 

 high spine to third sacral, which has a very much lower spine, 

 though it is still much compressed laterally. Six vertebrae take part 

 in the formation of the sacrum. The centra of the first few caudals 

 are flat with wide transverse processes, but these, as well as all the other 

 processes, gradually become suppressed and the neural arches disappear 

 so that the lower caudals are merely cylinders of bone. It is impossible 

 to determine the exact number of vertebrae taking part in the formation 

 of the tail, but it is fair to imagine that it had one at least as long pro 

 portionately as the horse. 



The scapula is remarkable for the persistence of the acromion process, 

 in which character it is unique among all Perissodactyls, with the excep- 

 tion of Pachynolophus (Orohippus) of the Bridger. The spine is better 



