NO. 2212. 



A NEW SPECIES OF EXTINCT HORSE— HAT. 



439 



The measurements of these teeth may be compared with those of 

 Equus hatcheri^ E. niohrarensis^ E. caballus^ E. francisci^^ E. lauren- 

 tius^ E. compUcatus, E. scotti, E. niohrarensis, E. laurentius^* E. 

 niohrarensis dlaskae.^ 



The teeth are remarkable on account of their relatively large size 

 and on account of the great breadth of the grinding surface as com- 

 pared with its length. As shown in the table on page 437, the whole 

 upper tooth line is equal to 37.4 per cent of the basilar length, while 

 in the other species there considered, and in E, caballtis, it lies be- 

 tween 33 per cent and about 34 per cent. The following tables show, 

 in various species, the widths of the premolars and molars as com- 

 pared with the length taken as 100 : 



Table of indices ofcrovms of upper teeth of extinct horaea. 



Table of indices of crowns of upper teeth of domestic horses. 



From the table showing the relation of the width of the crowns 

 to their lengths in the extinct species it is seen that the teeth of the 

 Yukon horse are much broader. It is known, however, that as the 

 upper teeth are worn down the fore-and-aft length of the crown 

 diminishes more rapidly than the width, with the result that the 

 index rises. The Yukon horse is older than any of the other extinct 

 species used in comparison. In the next table the upper teeth of the 

 Yukon horse are compared with those of domestic horses of various 

 ages. The indices of the horses in the American Museum of Natural 



iProc. U. S. Nat. Museum, vol. 48, 1915, p. 531. 



»Idem, p. 539. 



•Idem, vol. 44, 1918, p. 684. 



* Iowa Geol Snrv., vol. 23, pp. 160-208. 



8 Smiths. Misc. Coll., vol. 61, No. 2, p. 1. 



