South) Dakota School of Mines 95 



American Museum of Natural History has surpassed it. Gidley 

 stated three years ago, 1907, that the latter collection then con- 

 tained several thousand specimens — Eocene to Pleistocene, inclus- 

 ive. Granger, 1908, says that the Hyracotheres (Eocene) alone 

 were represented by several hundred specimens. Matthew and 

 Cook, 1909, add the information that in their recent work in the 

 Pliocene of northwestern Nebraska, they collected some hundreds 

 of incomplete jaws and about ten thousand separate teeth, be- 

 sides great numbers of limbs and foot bones. While it should 

 be borne in mind that the above collections represent to a large 

 extent fragmentary material, Osborn states, that in all the mus- 

 eums of the world there were in 1904 only eight complete 

 mounted skeletons of fossil horses, but that of these, five were 

 in the American Museum. 



The abundance of the fossil remains and their widespread 

 distribution geologically and geographically, clearly indicate that 

 for ages members of the horse family ranged over the country 

 in countless numbers. They were numerous in both North 

 America and South America. Beginning, as they evidently did, 

 in the earliest Tertiary or late Cretaceous in some generalized 

 form of small height, probably no greater, according to Marsh, 

 than a rabbit, they continued in increasing size to individuals 

 larger than the largest draft horses of the present day. The 

 earliest and the latest known members of the family do not 

 occur in the deposits described in this paper, but intermediate 

 forms are found in considerable numbers. These intermediate 

 forms merit our chief attention. 



All of the horses of the badland formations of the Black 

 Hills region had three toes on each foot.* Those of the older 

 formations, particularly of the Oligocene, stand approximately 

 midway in the genealogical line and show characters of absorb- 

 ing interest. 



The following species have been determined : 



Lower Oligocene. 



Mesohippus proteulophus Osborn. 

 Mesohippus hypostylus Osborn. 

 Mesohippus celer Marsh. 



*For sake of scientific accuracy it should be stated that this use 

 of the word! "Ibqrse" is an expression of relationship rather than a 

 name of specific or generic precision. It is, however, an expressive 

 term of much convenience and is often used) in the hest literature as 

 here given. 



