14 THsGITLATA. 



posteriorly, while the lower are concave. Incisors are always present 

 in the upper jaw, and may or may not he present in the mandible ; 

 in section, their dentine displays decussating striae. 



Genus MASTODON, Cuvier*. 



Including : — Trilophodon and Tetrahphodon, Falconer 2 . 

 Dibelodon and Tetrabecodon, Cope 3 . 



Dentition:— I. ^ y C. |j, Mm. \ Pm. [£=§|, M. | The upper 

 incisors are large, and may have longitudinal bands of enamel. The 

 lower incisors (when present) vary considerably in size, and are 

 always smaller than the upper. 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 alternate arrangement ; out- 

 lying tubercles may be present in the intervening valleys, which are 

 frequently entirely devoid of cement, and are never entirely filled by 

 it. The " intermediate " molars usually have an isomerous ridge- 

 formula, the number of ridges 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 the outer to the inner 

 side, and the reverse obtaining in the lower. Three cheek-teeth 

 may be in use at the same time. Premolars may or may not be 

 present ; and the crowns of the lower cheek-teeth are taller and 

 narrower than those of the upper. The cranium is usually less 

 elevated than in Elephas, and the mandibular symphysis may be 

 very long. 



The line of specialization in the dentition consists in the shortening 

 of the mandibular symphysis and disappearance of the lower incisors, 

 in the disappearance of the premolars, in the increase in the absolute 

 size and relative width of the cheek-teeth, and in the number and 

 height of their ridges, and the amount of cement. It is difficult to 

 say whether the form of tooth with continuous ridges, or that in 

 which the ridges are divided into distinct columns, is the more 

 primitive, as both types (M. turicensis and M. angustidens) occur in 

 the Middle Miocene, and both persisted into the Pliocene and Pleis- 

 tocene (M. americanus and M. arvernensis). M. turicensis passes, 

 however, into M. angustidens ; and it appears that, starting from the 

 latter generalized species, two branches of more specialized forms 



1 Ann. d. Museum, vol. viii. p. 270 (1806). 



2 Quart. Journ. Geol. Soc. vol. xiii. p. 313 (1857). As subgenera. 



3 Proo. Amor. Phil. Soc. vol. xxii. p. 5 (1884). 



