1881.1 11 I Cope. 



Before describing the Mexican species I make some notes on the others 

 embraced in the above list : 



Equus curmdens Owen. Of eight superior molar teeth from Buenos Ayres 

 in my collection, two second premolars are perfectly straight, while the 

 third true molar is the most curved. The other teeth exhibit different 

 degrees of curvature. The area of the anterior internal column is not so 

 flat on the inner side in any of them as in Owen's Plate (Voyage of the 

 Beagle, Vol. i). My teeth have also a rather greater transverse diameter 

 than Professor Owen's type. 



Equus caballus"L. The common horse differs from all ofthe extinct species 

 of the genus from American localities where the muzzle is known, in the 

 greater length ofthe latter, with its diastemata, of both jaws, and in the 

 greater prolongation of the maxillary bone posterior to the last true molar. 

 Appropriately to the anterior position ofthe molar series, the facial ridge 

 commences above the middle ofthe first true molar. In an Equus quagga 

 in my possession the ridge commences above the middle of the last pre- 

 molar. The basioccipital bone is more compressed than in any species 

 of the genus known to me. 



Equus occidentalis Leidy. This species is represented in my collections 

 by at least one hundred individuals, some of which have been lent me by 

 my friend Professor Thomas Condon of the University of Oregon. They are 

 nearly all derived from the Equus beds of the Oregon desert. Unfortu- 

 nately there is no perfect skull. A few specimens from the same region 

 I refer to the Equus excelsus, but as these are comparatively rare, I am safe 

 in referring most of the bones to the other species. In these I find the 

 following characters to separate the species from the fEqutis caballus : (1) 

 The basioccipital bone is not compressed, and besides its inferior lateral 

 angles it has a pair of lateral angles, one proceeding forwards from the 

 inferior border of each foramen condyloideum anterius. (2) The fossa en- 

 closed between the paroccipital process and the basioccipital, is deeper, 

 and has a raised border in front which separates it strongly from the plane 

 ofthe petrous bone. This is not found in E. caballus. I verify it in three 

 separate occipital bones of the E. occidentalis. (3) The astragalus and 

 other bones of the feet are smaller than in E. caballus; the first uamed 

 intermediate in size between that of the horse and that of the quagga. 

 The cannon bones, when of the same length, are more slender. (4) The 

 inferior canine issues in direct contact with the last incisor, without the 

 diastema seen in the horse ; and the incisive arc is narrower and more 

 produced. The symphysis is elongated not only forwards, but also pos- 

 teriorly. The mental foramen is anterior to the bifurcation of the rami 

 in E. occidentalis, posterior to it in E. caballus. 



Equus major Dekay. Dr. Leidy leads us to infer (Report U. S. Geol. 

 Survey Terrs., Vol. i, p. 244), that this species differs from the E. occiden-. 

 talis, in the generally greater complication of the enamel folds. This I 

 find to be the case in specimens from the Fish House, in the brick clay, 

 near Philadelphia, and from the Big Bone Lick, Kentucky. Leidy 



