58 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 20, 



Length of skull 



Extreme breadth of skull, at the zygomata 



Extreme breadth of cerebral part of cranium 



Breadth across postfrontal processes 



Breadth of upper jaw opposite first premolars 



Vertical diameter of skull opposite first true molar 



Vertical diameter of orbit 



From occipital crest to fore part of orbit 



From occipital crest to fore part of temporal fossa 

 From the fore part of the orbit to the end of nasals 



Length of mandible 



Length of symphysis mandibular 



Breadth of ascending ramus 



Height of ascending ramus at the condyle 



Height of ramus below first true molar 



Extent of molar series, upper jaw 



Extent of molar series, lower jaw 



Extent of three true molars, upper jaw 



Extent of the four premolars, upper jaw 



Extent of three true molars, lower jaw 



Extent of the four premolars, lower jaw 



Pliol 



ophus 



Hyracotherium 



vulpiceps. 



leporinum. 



in. 



lines. 



in. 



lines. 



5 











2 



2 







1 



3 







1 



6* 



1 



7 







9 







11 



1 



4 



1 



6* 







9 



1 







2 



9 







2 



2 







2 



3 







4 



4 







1 











1 



6 







1 



6 











7 







1 



11 



2 







2 















11* 



1 



1 







11* 







11 



1 











1 











Comparison of the Skull. — The extent and well-defined boundary 

 of the temporal fossae by the occipital, parietal, and post-frontal 

 ridges, and their free communication with the orbits, give almost a car- 

 nivorous character to this part of the cranium of Pliolophus : but, as 

 in the Hog, Hyrax, and Palseothere, the greatest cerebral expansion 

 is at the middle and toward the fore part of the fossae, with a con- 

 traction toward the occiput ; the brain-case not continuing to enlarge 

 backward to beyond the origin of the zygomata, as in the Fox. 



The zygomatic arches have a less outward span, especially at their 

 hinder pier, 27, than in the Carnivora. In this part of the cranial 

 structure Pliolophus resembles Palceotherium more than it does any 

 existing mammal ; but the post-frontal processes are longer and more 

 inclined backward. 



The incompleteness of the orbit occurs in both Anoplotherium and 

 Palceotherium, as in Rhinoceros, Tapirus, and the Hog-tribe ; but, 

 in the extent of the deficient rim, Pliolophus is intermediate between 

 Palceotherium and Tapirus. The orbit, PI. II. fig. 3, or, is not so 

 low placed as in Palceotherium, Tapirus, and Rhinoceros, nor so high 

 as in Hyrax or Sus. The straight upper contour of the skull is like 

 that in the Horse-tribe and Hyrax, and differs from the convex con- 

 tour of the same part in the Anoplothere and Palseothere. The size 

 of the antorbital foramen, PI. II. figs. 3 and 4, a, indicates no unusual 

 development of the muzzle or upper lip. In the conformation of the 

 nasal aperture by four bones (two nasals, 15, and two premaxillaries, 

 22), Pliolophus resembles the Horse, Hyrax, Hog-tribe, and Anoplo- 

 there, and differs from the Rhinoceros, Tapir, and Palseothere, which 

 have the maxillaries, as well as the nasals and premaxillaries, enter- 

 ing into the formation of the external bony nostril. 



The ungulate and herbivorous character of Pliolophus is most 



