1881.] 381 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 Chalicotheriida, and Hyracotherium 
in the Lophiodontide) would be necessary to obliterate this difference, 
is quite clear. The parent types of the order, which present the most 
generalized dentition, Hyracotherium, Rhagatherium, and Acoéssus, 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 (Hyracotheriing). 
A, No diastema behind first premolar. 
a, Third and fourth inferior premolar like the first true molar. 
Last inferior molar with five lobes....... siete viens ae .... Lophiotherium. 
AA, 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 asin Hyracotheriwm....... ee re eee ee Pliolophus. 
II. External lobes of superior molars flat, not well distinguished. 
(Lophiodontine.) 
“<4, No diastema in lower jaw. 
Last inferior molar with third lobe........... asracpeeiates esters 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 ............. dawBeereteis Bo Pachynolophus. 
Superior molars 6 ; 
Last inferior molar with heel..........-.--- ceceeerseveee. + Lophtodon. 
Last lower molar without heels, no horns ..... eehaiandsaedamstics Hyrachyus, 
Last lower molar?; ‘‘an attachment for a dermal horn on each nasal 
pone ’’........06s Diselearie wea da » alenoeate 1 eSeneses Seewee saws Colonoceras. 
The geographical range of these genera is as follows :— 
North America only........ eaoegsamesises .......Helaletes, Colenoceras. 
