1 9© I. J Gidley, The North American Species of Equus. 



1 03 



corresponding teeth is on the average greater than that between 

 the large and small varieties of E. cabailus, it would seem that 

 the teeth of the two lots could scarcely belong to the same species, 

 and although the character of size, alone, could hardly be con- 

 sidered sufficient ground for establishing a species, yet where this 

 difference exists, it seems reasonable to expect that when skulls 

 which represent such two lots of teeth are known, other differ- 

 ences will be found which will clearly mark them as distinct 

 species. 



It has been shown that the antero-posterior diameter of the 

 protocone is very little affected by wear; Table I (p. 98), makes it 

 clear, however, that the range of individual variability of this 

 diameter is very great, and c-annot be depended upon as a distin- 

 guishing character even in corresponding teeth of individuals of 

 the same size. 



The Lotuer Teeth. 



The characters of the lower teeth are, in general, affected in 

 the same way as the upper, and seem to be of even less value in 

 determining the species. A detailed discussion of the lower teeth 

 will therefore be omitted, except in regard to a character given 

 by Cope, that of the presence or absence of a cup in the external 

 incisors. An examination of the large amount of material at 



Fig. 5. External lower incisors showing development of cups. .-J, B, C, 

 E. caballus : D, E^ F, E. scotti. 



