268 ON THE EXTINCT MAMMALIA OF 



having existed at least three distinct species of Horse during the pliocene and post- 

 pliocene period of North America, — viz. : Equm fraternus, E. compUcatiis, and E. ex- 

 celsus and probably E. fossilis or E. cahallus may have existed in the extreme 

 north-west of the continent extending from Asia. 



The remains of Eqtms excehus in Dr. Hayden's collection from Nebraska consist of 

 fra«-ments of jaws with teeth, isolated teeth, and bones of the extremities, as follow : 



1. A fragment of the right side of the upper jaw, containing the back four molars, 

 from the Pawnee Loup branch of the Platte or Nebraska River. The jaw fragment 

 agrees in size and forni with the corresponding portion in the recent Horse. The 

 teeth are but moderately worn, and at the triturating surface together occupy a space 

 of about four and a quarter inches from before backward. As seen in figure 31, plate 

 XXI, the enamel folding surrounding the central lakes of the triturating surfaces is of 

 the simplest character. The absence also of the small inward folds above alluded to, 

 and usually observed in the recent Horse at the bottom of the deep oblique valley on 

 the inner side of the teeth, may be noticed, except in the last of the series, where it 

 exists. The measurements of the teeth are as follow :* 



Third molar, 

 Fourth molar, 

 Fifth molar. 

 Sixth molar, 



2. A fragment of an upper left molar, a third or fourth of the series, from the same 

 locality. It nearly agrees with that of the fossil just described. 



3. Two inferior molars, from the same locality. They are the fifth and the last of 

 the series, and are but little worn. They are represented in figure 39, plate XIX, 

 and exhibit no characteristic difierence from those of the recent Horse. Their mea- 

 surements are as follow : 



Length. 

 Inches. 



Fifth molar, . . . .31 



Sixth molar, . . . . 3i 



4. Seven lower molars, from the same individual, from the Niobrara River. They 

 consist of the second to the last inclusive of one side, and the posterior two of the 



* In the measurements of the teeth of equine animals in the succeeding pages, unless otherwise mentioned, 

 the following plan is pursued : 



In the upper molars the length is measured along the external median ridge of the crown, or when it is 

 mentioned internally, along the median column of the crown. In the lower molars, the length is measured 

 along the middle of the crown internally. 



