TEETH OF UNGULATA. 



3.53 



some distance from the molars, and creates a long diastema, as in 

 the Ruminants and Tapirs. The first deciduous molar is very 

 minute and is not succeeded by a premolar; yet, remaining longer 

 in place than the larger deciduous molars behind, it represents 

 the first premolar, and completes the typical number of that 

 division of the grinding series. If the dental formula of the 

 genus Equus be restricted to the functionally developed perma- 



nent teeth, it will be- 



3.3 1.1 3.3 3.3 



3.3' 



1.1 



3.3' 



3.3' 



40. 



The outer side of the upper molar of the Horse {Equus Ca- 

 ballus, fig. 269) is impressed, as in the Pala3othere, fig. 267, by 

 two wide longitudinal channels : the other evidences of the peris- 

 sodactyle type of grinding surface, and the modifications thereof, 

 are given at p. 341. In the lower jaw, the teeth, as usual, are 

 narrower transversely than in the upper jaw ; they are divided 

 externally into two convex lobes by a median longitudinal fissure, 

 and on the inner side they present three principal unequal con- 

 vex ridges, and an anterior and posterior narrower ridge ; but 

 the crown of the molar is penetrated from the inner side by 

 deeper and more complex folds than in the 

 Rhinoceros or Palaeothere. 



The incisors, figs. 280, 285, i, are arranged 

 close together in the arc of a circle at the ex- 

 tremity of both jaws. They are slightly curved, 

 longitudinally grooved, with long simple subtri- 

 hedral fangs tapering to their extremity, fig. 

 280. The crowns are broad, thick, and short. 

 The contour of the biting surface, before it is 

 much worn, approaches an ellipse. These teeth, 

 if found detached, recent or fossil, are distin- 

 guishable from those of the Ruminants by 

 their greater curvature, and from those of all 

 other animals by the fold of enamel (ib. c'), 

 which penetrates the body of the crown from 

 its broad flat summit, like the inverted finger 

 of a glove. When the tooth begins to be worn, 

 the fold forms an island of enamel inclosing Section of lncisor Horse 

 a cavity, s, partly filled by cement and partly by the discoloured 

 substances of the food ; this is called by horse-dealers the ' mark.' 

 In aged horses the incisors are worn down below the extent of 

 the fold, and the mark disappears. The cavity is usually obli- 

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



VOL. III. A A 



