VERTEBRATE FAUNA OF THE LOUP FORK BEDS. 33 



accuminate anteriorly. Excepting in this tooth and the last true molar, 

 the molars have at their rather advanced state of wear a greater transverse 

 than anteroposterior diameter. The last molar is strongly emai-ginate 

 posteriorly. In all, the protocone is more prominent inwards than the 

 hypocone. The enamel borders are everywhere perfectly simple. 



Measurements. 



Mm. 



Total length of incomplete skull 390 



Length from extremity of nasal bones to posterior border of orbit 265 



Length from extremity of nasal bones to posterior border of preorbital 



fossa 187 



Longitudinal diameter of orbit 60 



Longitudinal diameter of zygomatic foramen 62 



Vertical diameter of skull at front of orbit 110 



Length of dental series 15.5 



Length of true molars 7] 



Diameters of P. m.iii (anteroposterior 3-J 



( transverse 2C. 



Diameters of M. i j 



^. . c liT ■■■ S anteroposterior 



Diameters of M. in s ^ ^ 



( transverse 



transverse 30 



27 



20 



This species is only inferior in dimensions to the P. pachyops. The 

 molars liave a smaller anteroposterior diameter than in that species, and 

 the maxillary bone is produced much further posterior to them. The 

 wide difference in the preorbital region separates this species from the 

 P. fossulatus. Besides this, the muzzle is more produced, and the free 

 portion of the nasal bones is more elongate. 



The measurements of the teeth of this species agree sufficiently with 

 those given by Marsh for his Pliohippus pernix,* and the " deep irregular 

 fossa" in front of the orbit, and simple enamel ridges are found in both. 

 Marsh, however, says that the skull is " comparatively short," while that 

 of the present animal is elongate. If the P. mirabilis shall be ascertained 

 to have no lateral digits, it must be transferred, with the P. pernix, to the 

 genus Hippidium, Owen. 



The anterior part of the skull of a colt containing the entire temporarj- 

 molar dentition of both sides, presents the proportions of this species. 

 Their transverse diameter is 'smaller than in the worn temporary molar 

 of the P. perditus from Colorado already referred to, but this may be due 

 1 1 to a less degree of wear. As the wear has, however, progressed beyond 



the earliest stages, this may be questioned; but as the dimensions and 

 enamel pattern are appropriate, I refer it here provisionally. 



The protocone and hypocone are, in the first (anterior) molar, conflu- 

 ent with the paraconule and metaconule respectively. The same occuit 



* American Journal of Science and Arts, VII, 1874, p. 252. 

 3-Geol 



