1396 



CLASS MAMMALIA. 



alternate arrangement ; outlying tubercles may be present in the 

 intervening valleys, which are frequently entirely devoid of cement, 

 and are never completely filled by it. All the " intermediate " molars 

 usually have the same number of ridges ; this number varying from 

 three to five. The ridges are always bisected by a median cleft 

 traversing the long axis of the crown ; and the plane of wear of the 

 crowns of the teeth is oblique, that of the upper jaw inclining from 



Fig. 1268. — Mastodon angustidens. The fourth left lower milk-molar ; from the 

 Lower Siwaliks of India. 



the outer to the inner side, and the reverse obtaining in the lower. 

 Three cheek-teeth may be in use at the same time, and the sym- 

 physis of the mandible may be greatly elongated. The vertex of 

 the cranium (fig. 1267) is usually but slightly elevated, as in the 

 majority of Ungulates ; and in the less specialised forms (as may be 

 seen by comparing fig. 1267 with fig. 1270) the proportions of the 



Fig. 1269. — Mastodon angustidens. Vertical longitudinal section of the first lower true molar ; 

 from the Middle Miocene of France. Two-thirds natural size, b, Enamel; c, Dentine. (After 

 Gaudry.) 



entire skeleton depart less widely from the same type. The genus 

 has been divided into two groups, according to the number of ridges 

 in the cheek-teeth. In the first or Trilophodont group, the number 

 of complete ridges in each of the three " intermediate " molars is 

 three (fig. 1268) ; the ridge-formula of the complete series of cheek- 



