ORDER UNGULATA. 



1397 



teeth being represented by the ciphers Mm. 



M. 



3- 4 



1-2,3 3-3-4 



It will be noticed that in this section the number of ridges in the 

 third and fourth milk-molars and the first true molar is the same as 

 in Dinotherium. 



One of the earliest examples of this group is M. angustidens from the 

 higher Miocene of both Europe and India, of which two lower cheek- 

 teeth are represented in plan and section in figs. 1268 and 1269. The 

 simple structure of these teeth is shown in the section, where the valleys 

 separating the low, interrupted ridges, are seen to be devoid of cement, 

 although partially blocked by outlying tubercles. The imperfect fourth 

 ridge at the hinder extremity (right side of figures) of these teeth is 

 termed the talon. The mandibular symphysis (fig. 1267) in this species 

 was elongated and furnished with a pair of incisors ; while the milk- 

 molars were succeeded by premolars. Allied to this species are M. ftan- 

 dionis of the Upper Miocene and Pliocene of India, and M. pcntelici of 



[270. — Skeleton of Mastodon america?ius ; from the Pleistocene of Missouri, 

 Much reduced. 



the Lower Pliocene of Attica, Hungary, Samos, and Persia. From M. 

 angustidens there is but a step to the contemporaneous European M. 

 turicensis, in which the ridges of the molars extend straight across their 

 crowns, without outlying tubercles in the valleys ; and from this species, 

 again, the transition is easy to the Pliocene M. Borsoni of Europe, in 

 which the mandibular symphysis has become shortened in conse- 

 quence of the absence of lower incisors. A later offshoot from the same 

 branch is found in M. americanus (frequently known as M. giganteus, 



