ORDER UNGULATA. 



1395 



the simple marsupial dentition known in the whole class. In some 

 of those fossil forms in which the structure of the cheek-teeth is 

 simpler, there may be portions of three such teeth in use at the 

 same time. The serial position of individual teeth in any one 

 species can be more or less exactly determined by their absolute 

 size, and the number of transverse ridges which they carry. In all 

 members of the family there are never less than three ridges in the 

 fourth milk-molar and the first and second true molars (which are 

 often collectively spoken of as the " intermediate molars "), while 

 the last true molar has one or more ridges in excess of the preced- 

 ing tooth. Incisors are always present in the upper jaw of male 

 individuals, and may or may not be also developed in the lower 

 jaw T . In transverse section the dentine of these teeth displays a 

 peculiar arrangement of decussating lines, similar to the " engine- 

 turning " on the back of a watch-case, by the presence of which true 

 ivory can always be recognised. 



The earliest and least specialised genus of this family is Mastodon} 

 in which premolars and lower incisors are frequently present, and 



Fig. 1267. — Restoration of the skeleton of Mastodon angustidens. Greatly reduced. 

 (After Gaudry.) 



the upper incisors may have longitudinal bands of enamel. The 

 cheek-teeth carry transverse ridges, which may be entire or may be 

 divided into distinct inner and outer columns with a more or less 



1 Professor Cope proposes an arrangement of the Elephantidce differing from 

 the one generally adopted. Thus the genus Mastodon is split up into Tetrabel- 

 odon, in which there are both upper and lower tusks, and the former have an 

 enamel -band; Dibdodon, in which the lower tusks are usually wanting, but 

 the upper ones have an enamel-band ; and Mastodon, in which there are no lower 

 tusks, and no enamel-band on the upper ones. Elephas is divided into Emmen- 

 odon, typically represented by Elephas Clifti (apparently the type of Stegodon), 

 but also including E. flanifrons, and characterised by the presence of premolars ; 

 and Elephas, which is taken to include all the other species. 



