238 TEETH OF HORSE CHAP. 
the premolars, with the exception of the first, resemble the molars 
in their pattern. The orbit 1s completely surrounded by bone. 
The incisors are chisel-shaped, with a pit on the free surface. 
The canines are rudimentary if present. The radius and ulna 
are fused, as are the tibia and fibula. Although for the sake of 
uniformity a family, Equidae, is here separated from its allies, it 
is quite impossible owing to the full state of our knowledge of 
this group to draw a really hard-and-fast line between this family 
Fic. 124. —Side view of skull of Horse with the bone removed so as to expose the whole 
of the teeth. c, Canine; /’r, frontal ; 7}, 72, 7, incisors ; Z, lachrymal ; m1, m?, m°, 
molars ; Ma, malar or jugal; Wa, maxilla; Na, nasal ; oc, occipital condyle ; Pa, 
parietal ; pm, situation of the vestigial first premolar, which has been lost in the 
lower, but is present in the upper jaw ; pm", pm*, pm, remaining premolars ; Pda, 
premaxilla ; pp, paroccipital process ; Sq, squamosal. (After Flower and Lydekker.) 
and the Palaeotheridae. We shall deal presently with the con- 
jectured pedigree of the Horse, which naturally involves that 
family, and which presents an unbroken series from four-toed 
Perissodactyles to the present one-toed Horse, the various bones 
and teeth becoming modified in the course of the descent “ with 
the regularity of clockwork.” We are compelled to draw the 
line at functional second and third toes; directly these are no 
longer used the animal is a Horse in the strict sense! This is 
irrational and regrettable, but necessary for practical purposes, if 
