1S8J.] 1*^ [Cope, 



by Owen's E. conversidens, on which he relied to distinguish the species, 

 may be the result of distortion. The maxillary bones of the type are 

 loose and may be made to assume different angles to each other. The 

 last superior molar is represented as unusually short by Owen. This 

 appearance could be produced by the oblique angle of the aperture of the 

 camera in photographing, due to its too anterior position. Be that as it 

 may, I could detect no specific differences between the seven or eight spec- 

 imens I examined. 



The Equus tau is an average horse in all respects, presenting no very 

 tangible characters by which to distinguish it from the existing species of 

 the E. asimis and E. zebra group, so far as the parts which I examined go. 

 It has the internal anterior column of the superior molar always less in 

 diameter than half that of the crown of the tooth, and not characterized 

 by any marked peculiarity. The borders of the lakes have an entering 

 loop on each end of the inner border ; of these the adjacent ones are well 

 marked, and the remote ones little marked. External to the adjacent 

 loops the borders of the lakes are a little crenate. There is a small internal 

 median loop of the internal enamel border at the notch. The crowns of 

 the teeth are a little wider than long, and they are not curved. The pal- 

 ate notch reaches as far forwards as the posterior border of the second 

 true molar, and the palatal foramen is opposite the front of the third true 

 molar. The latter tooth is a little longer than the other true molars. The 

 second premolar is short and robust. The diastemata are rather short, as 

 can be seen by the appended measurements. 



Measurements. M. 



No. 1. Escuela des Minas. 



Length of precanine diastema 020 



Length of postcanine diastema 074 



Length of molar series 151 



No. 2. Museum Nacional. 



Diameters of P-m. ii -f anteroposterior 030 



t- transverse 024 



Tw. . ( QT) ...(anteroposterior 024 



Diameters of ?P-m. ni ■] ^ 



c transverse 027 



Diameter of ?Pm. iv 



anteroposterior 025 



transverse 028 



This species differs from the Equus andium "Wagn., so fully described 

 by Branco,* in the absence of a facial fossa. From Equus cahallus it 

 differs in the short diastemata, and the little posterior production of the 

 maxillary bone. How it differs from the species of the asinus section I do 

 not yet know. 



Equus excelsus Leidy, Extinct Mammalia Dakota and Nebraska, 

 1869, p. 266 ; pi. xxi, fig. 31. 



*Tn Dames and Kayser PahTeontologische Abhandlungen,1883, p. 110, Dr. Branco 

 furnishes reasons for believing tliat tlie E. argentinus Burm. is tlie same species 



