618 THE EOCENE FAUNA. 



Eocene period. They range from tlie 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 separated and little flattened ; lobes of infe- 



rior molars scarcely united [Hyrucotheriinw). 

 A. No diasteniata. 

 a. Fourth inferior premolar unlike the first true molar. 



Last inferior molar with tive lobes; superior premolars four Systemodon. 



AA. A diastema bcliind the first premolar and canine 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 Hynicotherium PUoloplnis. 



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



"J. No diastema in lower jaw. 



Last inferior molar witli third lobe - . Helaletes.''^ 



A A. 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 Heptoclon. 



Superior molars 6. 



Last inferior molar with heel Lophiodon. 



Last lower molar without lieel, no horns Hyrachyus. 



Last lower molar?; "an attachment for a dermalliorn on 

 each nasal bone " . . Colonoceras. 



The geographical range of these genera is as follows: ^k, 3i_ p^^.^ Jf riolt maxillary 



North America only Systemodon, Heptodoii, Hekdetes, ],oiie o^ Hepio^on sinnKhnisCoite; 



Colonoceras. from the AVasatcli beds of New 



North America and Europe . . Hyracliyus, Hyracotherhim, ^l^^'^^"' f''*'"' Captain Wlieeler's 

 T^,. , , leiiort, iv, ii, pi. Ixvi. 



Pliolophvs. 



Europe only Lophiodon. 



Five of the genera ascribed to North America have come under my observation. 



SYSTEMODON Cope. 



American Naturalist, 1881, ji. 1018. Paleontological Bulletin No. 34, p. 183. 



? 1 4 3 



Dental formula : I. - ; C. ; P-m. - ; M. — ; series in both iaws 

 3 1 4 ' 3 "^ 



continuous, or without distinct diastemata. Third and fourth superior pre- 

 molars with two external and one internal lobe. True molars with continu- 

 ous transverse crests; external cusps distinct, with regular lenticular section; 

 anterior external cingular cusp well developed. Inferior molars Avith unin- 



