454 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Hearing eruption. M2 also lies embedded, and nis 

 is beginning to form. 



Hatcher's original description of the skull is as 

 follows : 



When viewed from above the frontals are much the more 

 conspicuous elements. They are bluntly rounded posteriorly 

 and are projected far backward beneath and between the 

 lateral anterior projections of the parietals. Anteriorly the 

 frontals are continued into two long lateral projections which 

 extend beyond the orbits, overlie the posterior and lateral 

 margins of the nasals, and give rise to the pair of horn cores 

 that form such characteristic features in the Titanotheridae. 

 The nasals are arched superiorly, concave inferiorly, with 

 rather long posterior extensions interposed between the frontal 

 horns. In the present specimen the nasals are very thin along 

 their inner margins but much thickened externally and posteri- 

 orly, where they give the chief support to the horns. They 

 are slightly shorter than the premaxillaries and somewhat 

 emarginate anteriorly. 



The parietals are not so broad as the frontals. They are 

 deeply emarginate anteriorly and posteriorly, where they are 

 separated by the interparietal portion of the supraoccipital 



Figure 386. 



-Occiput of young skull of Brontops 

 brachycephalus? 



Carnegie Mus. 116. One-fourth natural size. The exoccipitals meet above the 

 foramen magnum. The wide supraoccipital affords attachment to the power, 

 ful ligamentum nuchae, the recti capitis lateralis, complexus, and other neol;: 

 muscles. 



much as in Equus. From the above description it will be seen 

 that the anterior border of the parietals overlies and incloses 

 laterally the posterior border of the frontals, while the anterior 

 border of the latter has a like articulation with the nasals, the 

 relative position of these bones being similar to that of the 

 shingles of a roof. 



The zygomata are rather broadly expanded and are composed 

 about equally of the malars and squamosals. 



When seen from the side the skull appears somewhat low, 

 with an abbreviated facial region and a rather long posterior 

 portion. The occipital crest and anterior frontal regions are 

 each somewhat elevated. The nasals appear rather deep and 

 send down the inferior and posterior projection, which articu- 

 lates by suture with the superior border of the maxillary. The 

 infraorbital foramen lies wholly within the maxiUary. The max- 

 illonasal suture is opposite the middle of the orbit. The lac- 

 rimal is rather large. The malar is long and thin; anteriorly 

 it has an extended contact with the maxiUary and posteriorly 

 with the squamosal portion of the zygoma. The squamosal 

 rises high above the external auditory opening and overlaps 

 the side of the parietal throughout most of its length. There is 

 a long, thin, transversely expanded postglenoid process and a 



shorter and proportionately stronger post-tympanic process of 

 the squamosal. The tympanic is absent, having been lost from 

 the present specimen. The periotic is present, and its para- 

 mastoid portion appears externally between the post-tympanic 

 and parocoipital process. Just above this there is another 

 small bone which is continued into a long pointed process 

 inserted between the exoccipital and the squamosal and pa- 

 rietal; it probably became coossified later with the periotic, 

 but in the present specimen it is seen as a separate bone, as 

 shown in Figure I and in Plate VII. [See figs. 385, 386.) 

 The exoccipitals are rather large and support the paroccipital 

 process and the occipital condyles. The latter are ossified 

 from two distinct centers, the articular portions bearing distinct 

 epiphyses, as shown in the accompanying figures. 



Seen from behind the occiput is low and broad. The condyles 

 are widely separated by the foramen magnum, which is much 

 broader than deep. The condyles are entirely supported by 

 the exoccipitals, which rise and meet in the middle line above 

 the foramen magnum, entirely excluding the supraoccipital 

 from any part in the formation of the superior border of that 

 opening. The supraoccipital is very broad and low. The 

 occipital crest is nearly flat above but broadly emarginate 

 posteriorly. (See fig. 1 of the text, and PI. VIII.) 



Inferiorly the palate is seen to be formed anteriorly by the 

 very short premaxiUaries, for the most part broken away in the 

 present specimen, and by the maxillaries, between the posterior 

 lateral extremities of which are inserted the palatines. These 

 form the posterior median portion of the roof of the palate and 

 send backward on either side a lateral projection along the inner 

 sides of the maxillaries and pterygoids, which are continued 

 nearly to the posterior end of the basisphenoid. 



The vomers are continued far back as a thin plate resting 

 upon the pre- and basisphenoids and sending downward a 

 thin, knifelike median bony septum. The basisphenoid in the 

 present specimen is entirely free from the basioccipital, the 

 suture being open, and the basioccipital had dropped out and 

 was lost before the specimen was found. The absence of the 

 basisphenoid and tympanic bones makes it impossible to describe 

 and locate the various foramina of this region of the skull. 



Stage 7. — Still more advanced is the stage shown in 

 Am. Mus. 497 (PL XXV, A), deciduous and permanent 

 upper teeth of Menodus giganteus. The first tooth of 

 the series is the permanent pS as shown by its exact 

 agreement in measurements and in characters with 

 the first premolar n adult skulls of M. giganteus (Am. 

 Mus. 505, 506). PhylogeneticaUy this tooth may have 

 been forced into association with the deciduous pre- 

 molars dp^, dp' through the abbreviation of the muz- 

 zle region and the consequent crowding backward of 

 the permanent canine. In this specimen the perma- 

 nent canine, which is still buried in the jaw, lies closely 

 appressed against the second permanent premolar. 

 The serial homology of this tooth as p^ is also estab- 

 lished by its measurements, as compared with the adult 

 Menodus giganteus. In this as in many other mammals 

 there was probably no deciduous predecessor of p^ 



In the specimen here figured m' is coming into 

 place. The identity of these teeth is also established 

 by comparison with the adult M. giganteus, so that 

 there is no doubt that dp*, dp', dp^ are correctly 

 identified. 



