HORSE. 573 



obliterated in the first or mid-incisors at the sixth year, in the second 

 incisors at the seventh year, and in the third or outer incisors at the 

 eighth year, in the lower jaw. It remains longer in those of the upper 

 jaw, and in both the place of the mark continues for some years to be 

 indicated by the dark coloured cement, as in the incisors (fig. 10) of 

 a Horse about sixteen years old. At this period the worn summits 

 of the incisors present a subtriangular form. 



The canines (figs. 9 & 10 c) are small in the Horse and rudi- 

 mental in the Mare ; the unworn crown is remarkable for the folding in 

 of the anterior and posterior margins of enamel, which here includes an 

 extremely thin layer of dentine. The upper canine is situated in 

 the middle of the long interspace between the incisors and molars ; 

 the lower canine is close to the outer incisor as in the Ruminants, 

 but is better distinguished by its cuspidate form. The representative 

 of the first premolar (figs. 1 & 6, j?, 1) is a very small and simple 

 rudiment, and is soon shed. The three normal premolars (^ 2, 3 & 4) 

 are as large and complex as the true molars ; the anterior one, 

 (p 2) is usually the largest of the series in the upper jaw, the anterior 

 lobe extending forwards into an obtuse angle. 



The upper molar teeth of the Horse present a modification 

 of the complex structure intermediate between the Anoplotherian 

 and Ruminant patterns. The crown is cubical, impressed on the 

 outer surface by two wide and deep longitudinal channels, penetrated 

 from within by a valley b entering obliquely from behind forwards 

 and dividing into, or crossed by the two crescentic valleys c & e, 

 which soon became insulated as in the Camel ; but a large internal 

 lobe ip) , at the end of the valley &, presents more of the Anoplo- 

 therian proportions than is shown by any Ruminant ; it is also at 

 first distinct, as in p 4, and though it soon becomes confluent with 

 the anterior lobe in the existing species of Horse, it continued 

 distinct much longer, and with more of the conical or columnar 

 form in the primigenial Horse (Hippotherium, fig. 3, ^) of the 

 miocene tertiary period. 



The molar teeth of the Horse have also, Cuvier remarks, a 

 closer analogy with those of the Rhinoceros than might at first be 

 supposed. The anterior crescentic enamel island (e) represents the 

 termination of the principal valley (6), which is cut off by a bridge 



