coFE.] PALEONTOLOGY MIOCENE PERIOD. 493 



period reminds one of the antelopes of the present period in Africa. It 

 is so distinct from the typical species of Symhorodon as to render it not 

 unlikely that it will be proper to call it Miobasileus hypoceras. 



HYEACODOiT, Leidy. 



Hyracodon nebrascensis, Leidy, Ext. Fauna Dak. and Neb., p. 232. 



Abundant in Colorado as in Dakota. During maturity, the first infe- 

 rior premolar is shed, while that of the upper jaw is retained, leaving 

 the formula f, |-. 



Hyracodon arcidens. Cope, Paleont. Bull., No. 15, 2. 



Established primarily on a specimen which includes the left maxillary 

 and prem axillary bones with the teeth as far jDosteriorly as the fifth 

 molar. Some of these were not fully protruded, and the third premolar 

 of the deciduous dentition was attached, the removal of which displayed 

 the crown of the permanent tooth. The species is about the size of the 

 S. nebrascensis, and differs in the form of the inner lobes of the molars 

 and of the fi.rst premolar. All the molars have the outer longitudinal 

 and inner transverse crests, the posterior short, the anterior much 

 curved backward round it, and thus forming the inner boundary of the 

 tooth-wall. The first premolar is shorter than the others, and has a 

 short anterior lobe. The milk-molars show more nearly transverse crests 

 as in Ehinocerus, but the first premolar had the anterior lobe. Canine 

 and first incisor short, conic ; second incisor with an outer lobe ; median 

 incisor transverse ; enamel smooth. 



Measurements. 



M. 



Length of four superior molars 0.072 



Length of diastema 006 



Length of canine and incisors 020 



Length of first premolar 014 



Width of first premolar 012 



Length of third premolar 021 



Width of third premolar 022 



Height of thii'd premolar 025 



ACERATHERIUM, Kaup. 



At least three species of this genus have left remains in the White 

 Eiver beds of Colorado, for the third, which I formerly referred to the 

 preceding genus, may find a more appropriate place here. They are 

 distinguished as follows : 



I. Crowns of premolars 2-3-4 broader than long : 



Smaller : symphysis mandibuli much shortened and contracted. — 



A. mite. 

 Larger : symphysis elongate, with large incisors. — A. occidentale. 



II. Crowns of premolars 2-3 as long as or longer than wide : 



Size of A. occidentale ; P. m., 2, subcuneiform. — A. quadriplicatum. 



ACERATHERIUM MITE, Cope. 



This species is hitermediate between the A. occidentale and Hyracodon 

 nebrascensis, not only in size, but in its short concave diastema, and 

 short, contracted symphyseal region. There are two large external 

 incisors, which are not only absolutely but relatively much smaller 



