32 GEOLOGICAL SURVEY OF TEXAS. 



Diameters of ISI. iii j anteroposterior 27 



I transverse 24 



Total length of molar series I45 



Lengtli of true molars qq 



The skull of this species indicates a species r.ather smaller than the ass. 



PROTOHIPPUS MIRAHILIS, Lcidj'. 



Elates VIII, IX, Figure 1 ; X, Figure 4. For synonymy above. 



As already remarked, this species was founded on a part of the max- 

 illary bone of a young animal which contains the last two temporary 

 molars. The crown of a permanent molar was removed from this frag- 

 ment and figured by Leidy. This fragment belongs to one of the largest 

 species of this genus, and it agrees in dimensions and in its large preor- 

 bital fossa with a nearly entire cranium obtained by Mr. W. F. Cummins 

 in Donley county. 



This cranium lacks only the anterior part of the premaxillary bone, and 

 the basioccipital. The parietal region is crushed on one side. But one 

 zygomatic arch is perfect. The size is about that of the ass. The an- 

 terior border of the orbit marks the posterior border of the last molar. 

 The malomaxillary ridge is prominent and acute, extending as far for- 

 wards as the infraorbital foramen, which is above the middle of the first 

 true molar. The preorbital fossa is large and deep, and is separated from 

 the orbit by a space two-fifths the width of the latter. In the P. sejunctus 

 this space is only one-fifth the orbital diameter, while in the P. fossulatus 

 the fossa is as far in advance of the orbit as its entire diameter. The 

 nasomaxillary fossa is distinct, is continuous posteriorly with the preor- 

 bital fossa, and extends anteriorly to the infraorbital foramen. The latter 

 becomes posteriorly a shallow fossa which extends anteriorly to above the 

 anterior part of the third premolar. A rather deep fossa behind the line 

 of the canine alveolus. The free portion of the nasal bones is acute, and 

 is relatively longer than in the type specimens of P. fossulatus. The 

 nasal fissure extends to above the anterior border of the root of the fourth 

 premolar. The maxillary bone projects posteriorly to the last molar once 

 and three-quarter times the diameter of the latter. The maxillary bones 

 are somewhat crushed together, so that the dimensions of the palate are 

 not determinable. The nareal notch extends to opposite the front of the 

 last superior molar. The posforbital bar is subtransverse, the free border 

 projecting forwards and outwards. The zygoma is short, its length equal- 

 ing the space from the superior border of the orbit to the posterior border 

 of the preorbital fossa. The glenoid surface is wide, and the postglenoid 

 process has a rather posterior position, and short diameter. Paroccipital 

 process lost. 



The fourth premolar has a single robust root. The third premolar is 



