100 ON THE EXTINCT MAMMALIA OF 



generally, but is absolutely smaller even than in the smaller species, 0. (jracUis. As 

 observed in the inferior view of the skull (Ancient Fauna of Nebraska, pi. iii, fig. 1 ; 

 pi. V, fig, 2), it appears as a cre.scentoid convex prominence, curving from within the 

 position of the post-glenoid tubercle to the root of the para-niastoid process. A ridge- 

 like prolongation, forming the posterior horn of the crescent, abuts upon the latter 

 process. A second ridge is directed outwardly backward and upward, forming the 

 other horn of the crescent, and corresponds with the vaginal process of the human 

 temporal. From the fore part of the bulla a shoit process projects exterior to the 

 eustachian orifice. Between the convex inner surface of the l)ulla and the basi- 

 occipital and basi-sphenoid there exists a wide reniform fissure, at the bottom of 

 which the periotic bone is visible. The external concave surface of the bulla includes 

 the space occupied by the styloid bone. 



The prominence of the auditory bulla of 0. Cidhert^07ii ordinarily does not project 

 nearly so low as the contiguous post-glenoid tubercle, and usually is not more than 

 half its bulk. In some instances the post-glenoid tubercle is proportionately smaller 

 and the auditory bulla may be larger, when the difference appears not to be so great, 

 though in all the skulls that have been examined the auditory bulla is considerably 

 smaller than the adjacent post-glenoid tubercle. 



The inflated portion of a full-sized auditory bulla in 0. Gulhertsoni measures five 

 lines antero-posteriorly, four and a half lines transversely, and three lines in depth. 



In 0. gracilis (Ancient Fauna of Nebraska, pi. v, fig. 3 ; pi. vi, fig. 2) the auditory 

 bulla is absolutely larger than in 0. Gulhertsoni, and exceeds in size the adjacent 

 post-glenoid tubercle, projecting inferiorly to about the same depth. In consequence 

 of its greater inflation than in the last-named species, the ridge abutting upon the 

 paramastoid process, and that corresponding with the vaginal process, appear much 

 less conspicuous. It forms an oval prominence, excavated postero-externally to 

 accommodate the styloid bone. Between its inner convex side and the basi-occipital 

 and basi-sphenoid there is a wide reniform interval, as in 0. Culherisoni. 



The inflated portion of the bulla measures, in a skull of 0. gracilis, five and three- 

 quarter lines antero-posteriorly, four and a half lines transversely, and three and a 

 half lines in depth. 



In 0. major, plate vii, fig. 1, the auditory bulla exceeds in size that of the Hog. It 

 is oval, with the long diameter antero-posterior, and slightly compressed at the sides. 

 Posteriorly it abuts directly against the root of the paramastoid process, and anteri- 

 orly against the ali-sphenoid. Internally its base rests against the edge of the 

 basi-occipital, the foramen lacerum occupying the interval between it and the 

 basi- and ali-sphenoid. It is more than twice the bulk of the contiguous post-glenoid 

 tubercle, and projects more than twice the extent inferiorly. It measures fourteen 

 and a half lines antero-posteriorly, eleven lines transversely, and nine lines in depth. 



