FOSSIL VERTEBRATES— MAMMALIA iJJ 



from the top of the ridges, leaving the softer dentine exposed, 

 which wore away more rapidly than the enamel and left strong 

 grinding ridges such as are found in the horse. With the 

 advance in the structure of the teeth appeared the degeneration 

 of the dentition as a whole, so that the modern elephant has 

 never more than one cheek tooth at a time in the jaw ; the teeth 

 appear successionally, the anterior one first and so backwards 

 during the life of the animal. 



Dinotherhim is the earliest known member of the group. It 

 was characterized by the development of a pair of down-curving 

 tusks in the lower jaw and a complete absence of incisors in 

 the upper jaw. There was a more or less complete dentition, 

 the jaw containing two premolars and three molars. The 

 single complete skull known is about three feet long. The 

 genus is known from the Miocene of Bohemia and from the 

 Pliocene of Central Europe and India. It is unknown from 

 America. 



Mastodon differed from the preceding in the presence of fewer 

 molar teeth and in the presence of tusks in both the upper and 

 lower jaws or in the upper jaw alone. The molar teeth exhibit 

 many variations in form, but in general the surface has a ten- 

 dency to be distinctly tuberculated, the transverse ridges multi- 

 plying and dividing into outer and inner halves. In size the 

 animal was nearly as great as the elephant. It was very com- 

 mon in the later Tertiary and forms have been discovered in 

 formations as early as the Middle Miocene in Central Europe. 

 In the Pliocene the genus seems to have reached a wonderful 

 development and to have ranged over the greater part of the 

 world ; forms have been discovered in nearly every part of the 

 world that man has visited. Near the close of the Pliocene it 

 disappeared from Europe, but is found in the Pleistocene of 

 America, sometimes associated with flint implements. 



Cakmvora. — The order Carnivora is one of the best known 

 from the fossil forms. In the earliest Eocene they approach the 

 Condylarthra to an extent that makes it difficult to tell the lines 

 of the Ungulates and the Unguiculates (claw-bearing forms) 



