I 20 Bulletin American Museum of Natural History. [Vol. XIV, 



Avere based on a series of specimens, casts, and photographs sent 

 to him by the Engineer of Mines, Mexico, he must have had 

 either the cast or the specimens themselves, upon which he 

 founded the species E. conversidens, and from which he took his 

 characters and measurements; for he mentions at the beginning 

 of his description (see p. ii8) that, among other features, the 

 curvature of the molar teeth corresponds closely with E. curvidens, 

 and this character could not possibly be made out from a photo- 

 graph showing only the crown view. 



Thus, then, the two characters above mentioned, together with 

 its smaller size and geographical location, seem to determine this 

 species fairly well. 



(13) Equus tau Owen. 



Synonym, Eqmis barcencBi Cope. 



Type Locality. — Valley of Mexico. 



Type. — A superior molar-premolar series lacking p^. 



Atiihors description. — " They [the teeth] are as much smaller than the cor- 

 responding teeth of Equus conversidens as are those of the A sin its fossilis 

 from the Oreston Cavern (History of British Fossil Mammals, p. 396, figs. 157, 

 158) compared with the teeth of Equus plicidens from the same cavern, and 

 they indicate a species about the size of the common Ass. 



" As compared with any of the smaller existing kinds of Equines the antero- 

 posterior diameter of the grinding-surface of the crown, especially in the pre- 

 molars, is in excess ; and in this character Equus tan also differs from Equus 

 conversidens, as it does in the greater relative size, especially antero-posterior 

 breadth, of the last molar m-^ : E. tau further differs in the greater flattening, 

 from without inward, of the inner lobeof most of the molars." 



Unfortunately Owen did not give any measurements with this 

 description, but if his figure of the type specimen is sufhciently 

 accurate to be relied upon, as seems quite probable from the 

 exactness of his other figures, the measurements may be given as 

 follows: 



p« 



( Antero-posterior diameter 22.5 mm. 



[ Transverse 



S Antero-posterior 



{ Transverse 



I Antero-posterior 



/ Transverse 



\ Antero-posterior 



\ Transverse 



j Antero-posterior 



\ Transverse 



19 



23 



20 



21 



19-5 



21.5 



iS 



22 



15.5 



