ELEPHANT AND MASTODON. 51 



The teeth of 31. Andium and M. angustidens are not included 

 in the plates of sections. Following the same serial arrangement 

 which has been observed throughout, their place would be between 

 M. Sivalensis and M. Ohioticus, the latter of which forms the ter- 

 minal link in the chain, establishing the nearest passage into Dino- 

 therium, and through that genus into the ordinary Pachydermata. 

 Figs. 11 and 12 of pi. 3, drawn of the natural size, represent 

 sections of the penultimate lower molar of two species of Dino- 

 therium, the former a fragment, showing the posterior half of the 

 tooth in D. Indicum, and the latter the whole tooth in D. gigan- 

 teum. The sections exhibit the same arrangement of the dental 

 substances as in M. Ohioticus. The tooth, fig. 12, consists of 

 two transverse crenulated ridges, and a talon ridge, while in the 

 equivalent molar of 31. Ohioticus, there are three principal ridges. 

 Corresponding to the smaller number of divisions, the ridges in 

 D. giganteum are more widely separated, less elevated, and 

 broader at their base, while the interspaces are also wider and more 

 open than in the North American Mastodon ; the layer of enamel 

 is of similar thickness, and there is no appreciable crust of cement. 

 The correspondence is followed out in the form of the sub- 

 ordinate heel ridge. D. Indicum is the species which is most 

 nearly allied to 31. Ohioticus ; and all the ascertained evidence 

 regarding it tends to prove that it belonged to a true Proboscidean 

 pachydermatous genus like the latter. 



§ III. — Ox the Succession of the Molars, and their Characters as indica- 

 ting Sectional Groups of Species. 



The molar teeth, developed during the course of life in the 

 ordinary Pachydermata, are divisible into three well-marked sets ; 

 the milk or deciduous molars, the false molars or successional 

 premolars, and lastly, the true molars. The milk teeth are so 

 distinct in their transitory character, from the permanent series, 

 that the consideration of the former is usually omitted in the con- 

 struction of generic definitions ; but in Mastodon and Elephas, 

 the succession of the teeth is so modified, and the premolars are 



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