1398 



CLASS MAMMALIA. 



or ohioticus) of the Pleistocene of North America. Remains of this 

 species (figs. 1270, 1271) are exceedingly abundant in the celebrated 

 "' Big-Bone-Lick" of Kentucky, in which region the species survived into 

 the human period. Although the mandibular 

 symphysis is short, a single small incisor is 

 occasionally found on one side of the jaw ; 

 premolars had however disappeared. 1 Other 

 Trilophodont species are M. Humboldti and 

 M. cordillerum {andiuui) of the Pleistocene 

 of South America, in both of which the trans- 

 verse ridges of the cheek-teeth are broken 

 into columns. The former species is distin- 

 guished by the short and edentulous sym- 

 physis of the mandible. According to Dr 

 Falconer, the latter had an elongated mandib- 

 ular symphysis and large lower incisors ; but 

 Dr Burmeister states that this reference is 

 incorrect, and that the symphysis was short 

 and tuskless. 



Fig. 1271. — Outer view of 

 second left lower true molar of 

 Mastodon americanus. Re- Jn ^ Tetralophodont grQup the num . 



ber of ridges in the cheek - teeth is 

 greater than in the former group, there being usually four in 

 the " intermediate " and five in the last true molars ; but there 

 may be occasionally five in the former and six in the latter. The 



complete normal ridge-formula is, therefore, Mm. — — -, M. ^~- •>. 



2-3-4 4-4-5 



In this group Mastodon arvemensis, from the Upper Pliocene of 

 Europe, is characterised by the number of accessory tubercles on the 



Fig. 1272. — Mastodon ar7<emensis ; the fourth left upper milk-molar ; from the 

 Norwich Crag. (After Lyell.) 



crowns of the cheek-teeth (fig. 1272) and the somewhat alternate arrange- 

 ment of the inner and outer columns of their transverse ridges, by which 



1 It appears probable that these teeth may occasionally be developed as an 

 abnormality. 



