clxxii GENERAL SUMMARY OF SCIENTIFIC AND 



which latter are now confined to the tropical parts of the so-called Old 

 World. In past periods, however, that family was well represented 

 within the present boundaries of the United States ; and at a still 

 more remote period i. e., in the Lower Miocene forms related to the 

 living type, but which were dilferentiated by quite distinct charac- 

 ters, and which were very closely related to at least the progenitors 

 of the recent Rhinocerotids, existed in numbers in what is now West- 

 ern America. This type, which has been distinguished as a family 

 by Professor Marsh under the name of Brontotheriids, was closely 

 related to the Rhinocerotids, but differed in details of dentition and 

 the feet, as well as the armature of the head. The typical species, 

 at least, were provided with horns, arising not from the median 

 line, as in the present Rhinocerotids, but from each side of the nose. 

 The brain was also very much smaller than that of the Rhinocero- 

 tids of the present epoch. According to Professor Marsh, four ge- 

 neric types can be recognized : these have been distinguished under 

 the names (1) Menodus or Titanotherium, (2) Megacerops^ (3) Bronto- 

 therium^ and (4) Diconodon. These are again combinable into two 

 groups, one composed of the first and second, and the other of the 

 third and fourth genera. All the known remains of the family have 

 been obtained from east of the Rocky Mountains, in the Lower Mio- 

 cene beds of Dakota, Wyoming, and Colorado. It is quite possible, 

 however, if not probable, that the family is identical with the Chali- 

 cotheriids which existed at nearly the same period in India as well 

 as in Europe. 



At an earlier epoch of the history of our continent (the Eocene) 

 existed a still more characteristic type, which has been isolated by 

 Professor Marsh as even a distinct order of Mammals, under the 

 name of Dinocerata. The animals so distinguished were closely re- 

 lated to the Proboscideans (elephants, etc.), but of a much more 

 generalized ty23e ; like them, they had columnar legs and pentadac- 

 tyle feet, but the articulations of the tarsals and carpals severally 

 with each other and with the proximal segments of the limbs were 

 quite diflerent. These animals, like the BrontotheriidcB^ were also 

 distinguished by their formidable armature ; some of them, at least, 

 are believed to have had as many as three pairs of horns two frontal, 

 two supraorbital, and two nasal. They were moreover provided with 

 very large and trenchant or sabre-like canines; the incisors were 

 rudimentary, or entirely obsolete ; the molar teeth were very small ; 

 the brain was still smaller than in the Brontotherium, although the 

 animals were in some cases at least as large as elephants. All the 

 certainly known representatives of this group have been found in 

 the Eocene beds of Wyoming. 



During the same ejDoch in North America lived another pecul- 

 iar type, and which has also been distinguished as a special or- 

 der of mammals by Professor Marsh under the name Tillodontia. 



