HOLLAND AND PETERSON: OSTEOLOGY OF THE CHALICOTHEROIDEA. 237 
origin, and slightly bent forward at their extremities. They are wider, though not 
relatively as long as in the horse. The basioccipital as shown on the under surface 
of the skull is flatter transversely than is the case in the genera Equus and Rhinoceros, 
but reveals on either side two slightly elevated longitudinal ridges. In the rhi- 
noceros there is a sharp median longitudinal keel; in the horse there are lateral 
ridges which unite on the median line before the anterior extremity of the bone. 
The posterior extremity of the basioccipital reveals an accessory condylar facet 
similar to that in the horse, but not extending so far forward as in the genus Hquus. 
The condylar foramen is quite large and located near the border of the accessory 
facet. 
The auditory bull, so far as is known from the specimens obtained by the 
Carnegie Museum and the University of Nebraska, were greatly inflated and lie 
on either side of the basioccipital and the posterior extremity of the basisphenoid. 
2 
a. 
Fic. 14. Superior view of posterior part of cra- Fig. 15, Diagram showing different locations of an- 
nium of M. petersoni (No. 133, Coll. of Harold J. terior margin of posterior nares. a, M. elatus; b, Nes- 
Cook). X#. toritherium; c, Macrotherium. 
So far as can be determined the auditory bulle were ovate and did not have the 
form of a bent cylinder as is the case in Macrotherium (cf. Filhol, ‘‘Mammiféres 
Fossiles de Sansan,”’ Plate XLV, Fig. 4). 
The meatus auditorius externus is lodged in a bony tube the outer end of which 
is expanded and lies just in front of the mastoid, which is relatively small and not 
nearly as strongly developed as in the horse or the rhinoceros. The tube narrows 
rapidly and passes in front of the anterior internal angle of the paroccipital, com- 
municating with the bulla at its posterior external angle, immediately behind the 
well-developed pit formed for the reception of the tympano-hyal. Immediately 
in front of this tube is a deep sulcus between it and the postglenoid process. 
