22 



THE HOKSE 



form, as compared with any modern ungulate of corre- 

 sponding dimensions. The hemispheres of the cerebrum 

 are especially small, compared with the cerebellum and 

 olfactory lobes. This is exactly in accord with what is 

 now generally known of the progressive diminution of 

 the size of the brain in all groups of animals the further 

 back we pass from the present time. 



The teeth were of the same number as in the great 

 majority of Eocene mammals — namely, three incisor or 

 front teeth, one canine or corner tooth, and seven cheek 

 teeth, consisting of four premolars and three molars, 1 or 

 eleven altogether on each side of the upper, and the 

 same number in the lower, jaw, making a total of forty- 

 four. These numbers are briefly expressed in the 



formula i. %c.^p.^ m. ^ = 11 x 2 = 44. This is an 

 o 1 4 o J 1 



extremely important formula to remember, as it is, as 



just mentioned, the one most commonly met with in 



mammals of the early Tertiary periods, and therefore 



the most generalised condition of dentition among all 



the higher orders of the class, and the one from which, 



by suppression or loss of certain of the teeth, all the 



special modifications now met with have been derived. 2 



1 The back teeth, grinding teeth, or cheek teeth are divided into 

 premolars and molars or true molars. In the ungulates there are 

 three or four of the former and always three of the latter, which are 

 the hindermost of the series and not preceded by milk teeth. 



2 In the pig and a few Insectivora alone among existing mammals 



