2 THE HORSE FAMILY. 



the enclosure of the socket of the eye by a complete bony ring is a 

 feature distinctive of the group. In all existing members of the 

 family, constituting the genus Equiis, there is only one toe on 

 each foot, although rudiments of lateral digits are represented 

 by the '' splint-bones '^ on each side of the upper end of the 

 cannon-bones. 



Dentition '^^^^ dentition of the Horse is illustrated on one side of 

 of the the table-case placed near the middle of the North Hall; 

 Horse. the object of the specimens being to show the alterations 

 which take place with age. In all its features the dentition 

 displays special adaptation for the masticating of vegetable food, 

 such as the herbage of the open plains upon which the species 

 dwells in a state of nature. The front teeth or incisors are, for 

 instance, used for cropping off the blades and stems of grass, 

 while the cheek-teeth (molars and premolars) serve for crushing 

 and breaking them into fragments. The tusks, or canines, so 

 greatly developed in carnivorous animals, are comparatively small 

 even in the males, and rudimentary in the females. 



The complete number of teeth in the addt Horse is that 

 characteristic of Ungulate or Hoofed Animals of the early Tertiary 

 period, viz.,thv^e incisors (i), one canine {c), four premolars (p), 

 and three molars (m) on each side above and below, or forty- four 

 in all. The first premolar {p. 1) is, however, very small, and 

 often wanting, especially in the lower-jaw; but instances of its 

 presence are shown m several specimens in the case ; these being 

 of interest, as remnants, on the point of disa})pearance, of a tooth 

 well developed in the Horse-like Animals of ancient times. 



The incisors, as mentioned above, have an infolding of the 

 surface, constituting a deep pit (the '^mark^^), a feature now 

 confined to the Eqiiidce. In consequence of this pit extend- 

 ing only a certain depth into the crown, it becomes obliterated 

 as the tooth wears away, so that its presence is a guide to the 

 age of the animal. The six principal cheek-teeth are in close 

 contact by broad surfaces fitting tightly against each other, so 

 that they collectively form one solid mass, presenting a grinding- 

 surface composed of substances of various degrees of hardness 

 (enamel, dentine, and cement), interwoven into an intricate 

 pattern so as to make most efficient natural millstones. The 



