270 TEETH OF THE RODENT MAMMALIA. 



growing molars are always more or less curved, for they 

 derive from this form the same advantage as the incisors, 

 in the relief of the delicate tissues of the active vascular 

 matrix from the effects of the pressure which would other- 

 wise have been transmitted more directl}^ from the grind- 

 ing surface; the capybara, and the Patagonian hare 

 {Dolichotis)^ afford good examples of this more complex 

 condition of the grinding teeth. 



The variety in the pattern of the folds of enamel that 

 penetrate the substance of the tooth, and add to its tritu- 

 rating power, is almost endless ; but the folds have always 

 a tendency to a transverse direction across the crown of 

 the tooth in the rodents. This direction relates to the 

 shape of the joint of the lower jaw, which almost restricts 

 it to horizontal movements to and fro, during the act of 

 mastication. In the true hoofed herbivorous animals, in 

 which the joint of the lower jaw allows a free rotatory 

 movement, the folds of enamel take other forms and 

 directions, with modifications, constant in each genus, and 

 characteristic of such. 



The horse is here selected as an example of such her- 

 bivorous dentition (Fig. 72). The grinding teeth are six 

 in number, on each side of both upper and lo\yer jaws, 

 with thick square crowns of great length, and deeply im- 

 planted in the sockets, those of the upper jaw being 

 slightly curved. When the summits or exposed ends of 

 these teeth begin to be worn down by mastication, the 

 interblended enamel, dentine, and cement show the pat- 

 tern figured in Cut 72 ; it is penetrated from within by a 

 valley, entering obliquely from behind forwards,- and 

 dividing into or crossed by the two crescentic valleys, 

 wliich soon become insulated. There is a large lobe at 

 the end of the valley. The outer surface of the crown is 



