556 II H I N O C E R S F N E 13 11 A S K A. 



RHINOCEROS NEBRASCENSIS. Xc'%. 



(Tab. xii. a, fig. (J; Tab. xii. b, fig. 5.) 



Rhinoceros Nebrascensis : Proc. Acad. Nat. Sci., vol. v., p. 121. 

 Acerotlicrium Nebra.sccnsc : ib., p. 331. 



This species was first established upon the anterior portion of a skull and lower 

 jaw, containing all the molar teeth of an old individual belonging to the collection 

 of the Smithsonian Institution. 



In Dr. Owen's collection is a head of the same species, of a very old individual, 

 with the upper part the Avhole length broken away. It contains all the molars 

 nearly perfect, which, however, have the crowns worn nearly to the edge of the 

 alveoli. 



There is also in the same collection a face, very much mutilated, except the fore- 

 head, of an individual which had just reached adult age. It contains all the 

 molars nearly perfect; the last one about two-thirds protruded. 



Besides the foregoing, there are several small fragments of upper and lower jaws, 

 as follows : 



1. A portion of an upper jaw, containing the finst permanent true molar, slightly worn, and a fragment 

 of the fourth permanent premolar still concealed within the jaw. 



2. A posterior fragment of a lower jaw containing the la.st three molar.s. 



3. A fragment of a lower jaw of a very young animal, containing tiie posterior temporary molar, and 

 the first permanent true molar, both unworn. 



Description of the head. — In the lateral view, Rhinoceros Nebrascensis presents 

 most of the peculiarities of Rhinoceros occidentalis. 



The root of the zygoma is implanted at the lower part of the middle of the tem- 

 poral fossa. The surface of the latter is convex, and relatively smooth. 



The orbit has nearly the same form as in Rhinoceros occidentalis, but its floor is 

 more superficial. Its entrance is well defined, as in the latter species, but the supra- 

 orbitar process is not quite so prominent nor so rough. The post-orbital process is 

 distinct. At tlie inner canthus is a short lachrymal process, and internal to this 

 two lachrymal foramina, one above the other. 



The face, as constituted by the alveoli for the superior molars, is vertical ; above 

 this it appears to have been oblique, Ijut it is too much broken in the specimens to 

 judge accurately of its form. 



The infra-orbital foramen is placed about an inch above the interval of the second 

 and third premolar. 



The malar bone is directed rather more outwardly than in Rhinoceros occiden- 

 talis, and its external face presents more upwards. 



The forehead is broad, elevated, and convex above the orbits, but depressed and 

 concave in the middle. The ridges upon the frontal bone which define the tempo- 

 ral fossa antero-superiorly, curve rapidly inwards from the post-orbital processes. 



