300 



MAMMALIA. 



skull and dentition of elephants are known. Dinotherium, 

 Mastodon, and Elephas follow each other in succession, and 

 show how the incisors immediately became the characteristic 

 elephantine tusks as soon as the proboscis had been ac- 

 quired, while the ordinary lophodont molars were more 

 gradually transformed, by complicated folding and deepening, 

 into the large powerful grinders of the Pleistocene and Recent 

 elephants. Dinotherium seems to have completely lost its 

 upper incisors (though this is not quite certain), while those 

 of the lower jaw form a unique downwardly curved pair of 

 tusks ; so that this genus cannot be regarded as an actual link 



Diagrammatic vertical longitudinal Sections of Molars of Proboscidea. 



A. Dinotherium. B. Mastodon, c. Elephas (Loxodon). D. Elephas (Eu- 



elephas). c. cement; d, dentine; e, enamel; p, pulp-cavity; w, worn front 

 portion of tooth. 



between Elephas and its primitive ancestors. But some of the 

 Miocene species of Mastodon exhibit an upper and lower pair 

 of persistently growing functional incisors, very suggestive 

 of the front teeth of an ordinary rodent ; and at least the 

 upper pair usually retain a longitudinal band of enamel, which 

 is slightly disposed in a spiral direction, though evidently 

 reminiscent of the time when it was a regular straight band 

 to ensure a chisel-like edge as it worked against its fellow in 

 cutting food. Most of the Pliocene species of Mastodon advance 

 a grade further and are entirely destitute of the lower incisors, 

 while the upper pair are tusk-like, usually with no traces of 

 the enamel-band ; and Elephas, as is well known, retains only 





