1881.1 ''^^ I Cope. 



points of the separation, or non-separation, of the exterior crescents of the 

 superior molars, as already pointed out. That no great modification of 

 known forms (as Lambdotherium in the GJialicotherndm, and Hyrncotherium 

 in the LopModontidce) would be necessary to obliterate this difference, 

 is quite clear. The parent types of the order, which present the most 

 generalized dentition, Hyracothermm, Rliagatherium, and Acoessus, were 

 cotemporaries of the Lower Eocene epoch. 



LOPHIODONTID^. 



This family embraces a larger number of known species than any of 

 the others of the order. With one exception, all the species belong to the 

 Eocene period. They range from the size of a rabbit to that of an ox. 

 They resembled most, among living animals, the tapirs. 



The genera are characterized as follows : — 



I. External lobes of superior molars well separated and little flat- 

 tened ; lobes of inferior molars scarcely united (Hyracotheriince). 



A. No diastema behind first premolar. 



a. Third and fourth inferior premolar like the first true molar. 

 Last inferior molar with five lobes LopMotherium. 



A A. A diastema behind the first premolar in both jaws. 



a. Last inferior premolar different from first true molar ; 

 Last inferior molar with heel ; cross-crests of superior molars interrupted ; 



Hyracotherium. 



aa. Last inferior premolar like first true molar ; 

 True molars as in Hyracotherium Pliolophus. 



II. External lobes of superior molars flat, not well distinguished. 

 {Lophiodontince. ) 



"J. No diastema in lower jaw. 

 Last inferior molar with third lobe Helaletes. ' ' 



AA. Lower jaw with diastema. 



*No diastema behind first premolar. 



a. No inferior premolars like the true molars. 



Superior molars 7. 

 Last inferior molar with heel PacJiynolopTius. 



Superior molars 6 ; 



Last inferior molar with heel Lophiodon. 



Last lower molar without heels, no horns Hyrachyus. 



Last lower molar?; "an attachment for a dermal horn on each nasal 



bone " Golonoceras. 



The geographical range of these genera is as follows : — 

 North America only Helaletes, Golonoceras. 



