GEOLOGICAL SURVEY OF THE TERRITORIES. 361 



rope, Asia, and North America. The skeleton was constructed on the 

 same general plan as in the preceding genera, but in addition to the 

 well-developed toe or hoof, which supported the animal, it possessed an 

 additional pair of toes to each foot. These, however, were not suffi- 

 ciently developed to touch the ground, but projected behind and later- 

 ally like the rudimental toes of hogs and ruminants. The arrangement 

 of the enamel in the molar teeth is of a more complex character than 

 in Equus and Protohippus. A species is indicated by remains found in the 

 Ashley Eiver deposits of Charleston, South Carolina. It was not more 

 than about half the size of the ass, and has been named Hipparion 

 venustum. 



Hipparion occidentale. — This species, about the size of an ass, is indi- 

 cated by remains which were found in the mauvaises terres of White 

 Eiver, Dakota; but which clearly do not belong to the miocene forma- 

 tion of that region. They pertain to a superficial portion of the tertiary 

 deposit, cotemporaneous with the pliocene deposits of the Niobrara 

 Eiver. 



Hipparion speciosum. — A supposed second species, nearly the same 

 size as the preceding, is indicated by molar teeth found at Bijou Hill, 

 on the Missouri Eiver, and in the pliocene sands of the Niobrara Eiver. 

 A fragment of a tooth, likewise apparently belonging to this species, 

 was found in digging a well, at the depth of forty feet, in Washington 

 County, Texas. 



Hipparion affine. — A third species, also about the same size as the 

 preceding, is indicated by teeth discovered by Professor Hayden in the 

 pliocene sands of the Niobrara Eiver. The enamel folding of the upper 

 grinders is of a simpler character than in the former species. 



Hipparion gratum. — This species, based upon a number of teeth 

 obtained by Professor Hayden in the pliocene sands of the Niobrara 

 Eiver, was considerably smaller than the other species with which it was 

 associated. 



MERYCHIPPUS. 



Another extinct equine genus, has permanent molar teeth like those 

 of Protohippus, but the skull presents deep lachrymal depressions in 

 advance of the orbit, as appears also to be the case in Hipparion, but 

 which, in the former, as in the true horse, are absent. The temporary 

 teeth of Merychippus resemble more nearly in appearance the permanent 

 ones of the members of the next family than they do those of the other 

 known equine genera. It was from the remarkable resemblance of 

 these to the large grinders of ruminating animals that led to the adop- 

 tion of the name of Merychippus, which literally signifies ruminating 

 horse. The name, however, refers to the resemblance alone, and does 

 not indicate that the animal partook of the peculiar habits of the 

 ruminants. 



Merychippus insignis. — This species, about the size of the ass, was 

 originally established on a jaw fragment, containing teeth, which was 

 discovered by Professor Hayden at Bijou Hill. Many additional re- 

 mains, referable to the same species, were subsequently found in the 

 pliocene sands of the Niobrara Eiver, during Warren's expedition in 

 1857. 



Merychippus mirdbilis. — This second species, rather larger than the 

 preceding, is indicated by remains discovered by Professor Hayden 

 in the pliocene sands of the Niobrara Eiver and on Little White Eiver, 

 Dakota. 



