PERISSODACTYLA. 335 



represented in the Eocene Tertiary, while tlie general 

 structure was precisely that of Palaiothermni, the prgemolars 

 exhibit a simpler type of structure than the true molars, 

 while the first pra:;molar is absent. 



Through Anchitherium the Pcdceotheridce are brought so 

 close to the Eqiddm, that it seems probable that the two 

 possessed a common origin. 



Fam. 6. Macrauchenidm. — It seems necessary in the mean- 

 while to establish a separate family for the reception of the 

 curious but still imperfectly known Mammals which con- 

 stitute the genus 3facrauchenia, the remains of ■which have 

 been discovered in the Pliocene or Post-Pliocene deposits of 

 the South American continent. Owing to the fact that the 

 transverse processes of the cervical vertebrae are without a 

 foramen for the passage of the vertebral artery, this anom- 

 alous genus was originally referred to the Ccmielidce, and 

 named accordingly. It is quite certain, however, that the 

 genus must be placed among the Perissodactyles, as all the 

 feet are three-toed, there is a third trochanter to the femur, 

 and the astragalus resembles that of the Odd-toed Ungulates 

 in having no articular facet for the cuboid bone. The dental 

 formula is — 



.3—3 1—1 5—5 3—3 



i ; c ; pm ; m = 46. 



3—3 1—1 ^ 4—4 3—3 



The teeth form a nearly continuous series ; the canines are not 

 excessively developed ; the incisors have a deep coronal 

 pit (as in Equus) ; and the lower molars resemble those of 

 PalceotJicrimn in being doubly crescentic. The general form 

 of the skull resembles that of the Horses. 



Fam. 7. Fquidce {Solidungulcc or Solipedia). — This family 

 comprises only the existing Horses, Asses, and Zebras, 

 usually comprised in the single genus Equus ; but along 

 with these we must place a large number of extinct forms, 

 in many cases of an extremely interesting character. In all 

 the recent forms, one of the most striking features is the 

 reduction of the digits to a single perfect toe (the third) on 

 each foot; but this character is not available in character- 

 ising the family, since various fossil types show an increase 



