UNGULATA. 303 



giffanteum, with a skull about a metre in length, from the Lower Pliocene 

 of Hesse Darmstadt. Remains of this and other smaller species occur 

 widely distributed in the Middle and Upper Miocene deposits of central 

 Europe. Similar teeth are met with in the Siwalik Formation (Lower 

 Pliocene) of India; while limit-bones from the Pliocene of the Island 

 of Sumo* ami from Pikermi, near Athens, have also been ascribed to the 

 same genus. No evidence of Dinotherium has hitherto been discovered in 

 America. 



Mastodon (tigs. 171 B, 174, 175). The 'Proboscideans commonly 

 grouped under the generic name of Mastodon differ from Dinotherium in 

 their somewhat larger and more complicated molars, which are never all 

 simultaneously in use. Whereas the number of ridges in the six teeth 

 (three milk-molars, three true molars) of Dinotherium is expressed by the 

 figures 2, 2, 3, 3, 2, 2 ; in the corresponding teeth of Mastodon the 

 numbers are either 1 (or 2), 2, 3, 3, 3, 4 or 2, 3, 4, 4, 4, 5, with very little 



FIG. 173. 



l>iii<itlii'rinm i/ii/nitti'iim; left upper dentition, one-quarter nat. size. M. 

 Miocene ; Gers, France. The two teeth to the left of the figure are the 

 third and fourth premolars, while the other teeth are the three true molars. 

 (After Gaudry.) 



variation. There is also nearly always a tendency in Mastodon for each 

 ridge to divide into right and left halves ; while very frequently these 

 ridges are further subdivided into nipple-like bosses (hence the generic 

 name), which are sometimes displaced and partly fill the valleys. The 

 enamel on the teeth is very thick, and cement is either absent or very 

 little developed. There are always three milk-molars in both jaws, and 

 of these the two hiiulermost are usually replaced by premolars, though in 

 some of the later species (e.g., Mastodon americamus or M. ohioticus) they 

 persist throughout life. Some of the earlier species bear tusks in both 

 jaws, and exhibit a much-produced mandibular symphysis (Tetrabelodon) 

 others have only upper tusks, but these retaining some enamel (Dibelodori); 

 while others possess only upper tusks without enamel (Mastodon proper). 

 Again, in some species, as already indicated, the three "intermediate 

 molars" (namely, m-m. 4, m. 1, m. 2) each bear three ridges, while in 



