572 UNGULATES- 



like the last trilobate deciduous lower molar of the Hog it increases 

 in thickness posteriorly, instead of diminishing here like the last 

 true molar of the lower jaw of the adult Hippopotamus. 



ANISODACTYLE PACHYDEKMS. 



210. Equidce. — The Horse will yield us the first example of the 

 dentition of the hoofed quadrupeds with toes in uneven number, 

 because it offers in this part of its organisation some transitional 

 features between the dental characters of the typical members of 

 the Isodactyle and of those of the Anisodactyle Ungulata. 



All the kinds of teeth are retained and in almost normal 

 numbers in both jaws, with as little unequal or excessive develop- 

 ment as in the Anoplothere ; but the prolongation of the slender 

 jaws carries the canines and incisors to 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, as 

 in the Anoplothere, by a permanent 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 permanent teeth, it 

 will be : • 



Incisors ; — ; canines — ; premolars — ; molars — : = 40. 



The incisors are arranged close together in the arc of a circle 

 at the extremity of both jaws ; they are slightly curved, with long 

 simple subtrihedral fangs, tapering to their extremity. 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 distinguishable from those of the Ruminants by 

 their greater curvature, and from those of all other animals by the fold 

 of enamel, (PL 136, tig. 11, a) 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 a cavity {a, figs. 8 & 9) partly filled by cement, and partly 

 by the discoloured substances of the food, and is called the ' mark.' 

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

 fold, (fig. 11, (() and the 'mark' disappears. The cavity is usually 



