xni PHYLUM CHORD ATA 649 



In the Perissodactyla the molars and pre-molars form a con- 

 tinuous series of large teeth x with ridged or complexly-folded 

 crowns, the posterior pre-molars often differing little in size and 

 structure from the molars. In the Horse (Fig. 1205) the formula is 

 .314 3 

 *-3' C 'l'^4' m -3 ==44 ' 



but the first pre-molar is a small tooth which soon becomes lost, 

 and may belong to the milk-dentition. A fold of the enamel 

 dips downwards (i.e. towards the root) from the extremity of the 

 incisor teeth like the partly inverted finger of a glove ; the canines 

 are small in the female, and may not appear on the surface. There 



Til,* 



Fia. 1205. Side view of skull of Horse with the bone removed so as to expose the whole of 

 the teeth, c. canine ; Fr. frontal ; i 1 . f 2 . >. incisors ; L. lacrymal ; Ma. jugal ; MX. maxilla ; 

 m 1 . 2 . 3 . molars ; Na. nasal ; oc. occipital condyle ; Pa. parietal ; p. m l . situation of the 

 vestigial first pre-molar, which has been lost in the lower, but is present in the upper jaw ; 

 pm 2 . pm*. pm+. remaining pre-molars ; PMx. pre-maxilla ; pp. par-occipital process ; Sq. 

 gquamosal. (After Flower and Lydekker.) 



is a wide interval in both jaws between the canines and pre-molars. 

 The pre-molar and molar teeth present a complicated pattern due 

 to folds of the enamel, which differ in their arrangement in the 

 upper and lower jaws ; their roots become completed only at a 

 late period. 



In the Hyracoidea the dental formula is 



.104 3 



The upper incisors are not unlike the larger pair of the Rabbit in 

 shape, though prismatic and pointed, instead of compressed and 

 chisel-like ; they grow from persistent pulps. The outer incisors 



