10 INTRODUCTION. 



in others still, they agree with tlie Proboscidians. The points of similarity, 

 however, are in most cases general characters, which point back to an 

 earlier, primitive, ungnlate, rather than indicate a near affinity with 

 existing forms of these groups. This subject will be more fully discussed 

 in the concluding chapters of the present memoir. 



The Dinocerata, so far as now definitely known, may be placed in 

 three genera, Dinoceras, Marsh, Thioceras, Marsh, and Uiutatherium, Leidy. 

 The type specimen of TJintathenum was discovered near the base of the 

 series of sti'ata containing the remains of the Dinocerata. Dinoceras, so 

 far as known, occurs only at a higher horizon, while Tinoccras has "been 

 found at the highest level of all. The characters of these three genera 

 correspond in general with their geological position. Uintafherium 

 appears to be the most piimitive type, and Tinoceras the most specialized, 

 Dinoceras being an intermediate form. The material at hand for 

 determining the characters of the two latter forms is abundant, but in 

 regard to Uintatherium, some important points relating both to the skull 

 and skeleton still remain in doubt. 



The number of species of the known Dinocerata is a difficult matter to 

 determine, especially as the limitaticms between species are now generally 

 regarded as uncertain. About thirty forms, more or less distinct, are 

 recognized in the Synopsis at the end of the volume. The number might 

 easily be increased, if fragmentary specimens were used as the basis for 

 specific names. 



