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y a pair of parallel grooves (Fig. 2.6: g1), that lead out from the posterior carotid
foramen (Fig. 2.6: pcf). These grooves lose definition where the posterior septum
contacts the promontorium, but another groove becomes visible more anteriorly on the
lateral aspect of the promontorium itself. This grooves wraps ventromedially around the
promontorium and approaches the s1 (Fig. 2.6: g2). This groove is present in many
specimens, recognizeable by the relationships just described and is referred to as g2. It is
also often present as a pair of parallel grooves, although not in UALVP 49105. A more
dorsolaterally directed groove (Fig. 2.6: g5) that approaches the epitympanc crest, a ridge
of bone that extends laterally from the promontorium, just anterior to the fenestra
vestibuli (Fig. 2.6: ec). The g5 groove tends to be more broadly excavated than the g2
groove.
As implied in the discussion above, the remnants of s1 and s2 are preserved, as
well as s3, cochlear canaliculus and posterior septum. Furthermore the posterior wall of
the bulla is preserved where it contacts the posterior septum, and thus a fragment of the
internal carotid canal is preserved. Although the ventral margin of the canal is broken
off, its mediolateral diameter measures 0.29 mm (Fig. 2.6I). The medial tympanic
process of UALVP 49105 is broken, but clearly shows a dorsal and ventral lamina of
bone comprising it (Fig. 2.6: 25-26). Again, because of the historical prediction that the
plesiadapid bulla has an entotympanic element in it (e.g., MacPhee and Cartmill, 1986), it
is tempting to conclude that these represent two distinct bones. As with the skull, HRxCT
does not reveal differentiation between these two layers (Fig. 2.6B), even though this
isolated specimen was scanned at a resolution five times higher than the skull (8 m vs.
40 m). The semicircular canals of the pars canalicularis surround a deeply excavated
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