Excerpt from Journal of Morphology, 1891, Vol. 5
This shape of the ali-sphenoids does not occur in the recent species, as in correlation with the heightening 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 tympanics are inflated into enormous bullae, which in both species of Pcebrotherium 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 shallower 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 upwards 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 narrowed 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 posteriorly 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.
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