440 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 53. 



History are derived from measurements furnished by Gidley.^ Not 

 even in the horse supposed to be 20 years old are the teeth as broad 

 relatively to the length as in the Yukon fossil. However, in a horse 

 in the U. S. National Museum (No. 5044), the first and second 

 molars exceed very slightly in relative breadth those of the Yukon 

 horse; but that horse is evidently much older; or, at least, the 

 teeth are much more worn. The cups are gone from the upper in- 

 cisors and the last premolar is worn down to a height of only 

 26 mm. 



When we come to compare the lower teeth with those of the sev- 

 eral extinct species, with those of a large horse about 8 years old, 

 and with the old horse just mentioned, it is foimd that none of them 

 approach in width the teeth of the Yukon horse. 



Table of indices of croivns of lower teeth of horses. 



The grinding faces of the upper teeth (pi. 57) present various 

 peculiarities. The outer styles are only moderately developed. As 

 compared with the domestic horse, the outer face of the paracone 

 is shallow and flat. The fossettes are extremely narrow. This is 

 not due wholly to age, for those of older horses are wider. As a result 

 of this narrowness the areas representing the other elements of the 

 teeth are broader than usual. The protocones are unusually long 

 and they are thin from side to side? Their lengths are given among 

 the measurements of the upper teeth. In some cases the anterior 

 end comes into contact with the enamel of the next tooth in front. 

 The post-protoconal valley is directed strongly forward; and it lacks 

 much of attaining the center of the crown. In each premolar there 

 is an indication of a reentrant fold near the head of the valley, but 

 none in the molars. Probably at an earlier stage of wear this fold 

 was deeper, but in the premolars of a much older domestic horse the 

 valley has the fold very distinct. 



The enamel surrounding the fossettes is extremely simple (pi. 57, 

 figs. 2, 3) ; but here again it is necessary to make some, but not too 

 much, allowance for age. Even the very old domestic horse referred 

 to has the enamel more plicated. 



As shown in a table given above, the lower teeth (pi. 58, fig. 2) 

 likewise are unusually broad. The outer valley does not in any of 



iBolL Amer. Mus. Nat Hist., vol. 14, p. 98. 



