No. i.] OSTEOLOGY OF PCEBROTHERIUM. 15 



the ali-sphenoids is rather small, and on each side of the basi- 

 sphenoid they flatten out into broad surfaces which slope gradu- 

 ally to the glenoid cavities, the latter being but little above the 

 level of the cranial axis. This shape of the ali-sphenoids does 

 not occur in the recent species, as in correlation with the height- 

 ening of the molar crowns the glenoid cavity is placed at a 

 higher level, so that this portion of the ali-sphenoids has become 

 almost vertical. The orbito-sphenoids seem to have about the 

 shape and proportions seen in the llama. 



The tympanies are inflated into enormous bullae, which in both 

 species of PcebrotJierium are relatively much larger than in the 

 recent genera, and are more rounded. In the small species, 

 P. Wilsoni, they are larger and less compressed than in P. labi- 

 atum, and in both the long diameter is directed nearly parallel 

 to the cranial axis, while in the modern forms it is placed at a 

 wide angle with it. The postero-external angle of the bulla 

 shows a deep styloid groove, " a vertical gutter ending below in 

 a pit with a process for the conjunction of the styloid bone" 

 (Leidy, No. 19, p. 143). The pit is decidedly smaller and shal- 

 lower than in Auchenia, and is less distinctly separated from the 

 groove. The stylo-hyal does not appear to have been anchylosed 

 with the tympanic ; at all events, I find no trace of it in any 

 of the specimens which I have examined. As in the Tylopoda 

 generally the bulla is filled with cancellous bony tissue. The 

 external auditory meatus is a closed ring, opening slightly up- 

 wards and backwards; its rim does not project at all beyond the 

 surrounding parts of the squamosal. The mastoid portion of 

 the periotic is exposed on the occipital surface, filling a very 

 narrow space between the squamosal and the ex-occipital. 



The parietals are very long, and form almost the whole of the 

 roof of the cranium, but for most of their length they are nar- 

 rowed by the great extension of the squamosals, in front of 

 which they send down very long descending processes to meet 

 the orbito-sphenoids. Throughout their entire length they unite 

 to form the low and very thin sagittal crest which rises poste- 

 riorly to join the crest of the inion. Anteriorly the parietals 

 diverge to receive the pointed ends of the frontals, and are thus 

 much shorter in the median line than on the sides. This 

 the cranium is very like the corresponding part in the 

 it the sagittal crest is not so prominent and the parietal 



