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is better preserved on the left side than the right side, and the description of its
morphology is therefore based on the left. The maximum diameter of the aperture for the
fenestra vestibuli, or “oval window,” is 1.32 mm (Fig. 3.9: av). The spiral cochlea
measures 21.03 mm in length. The width of the pars cochlearis of the right side is 4.43
mm, while its dorsoventral depth, as measured from the endocranial surface, is about 5.5
mm. The ventral surface of the pars cochlearis (the promontorium) is criss-crossed by
grooves of varying diameter (Fig. 3.9). Several bony ridges (or septa) extend out from
the promontorium at varying angles, some of which are only apparent with HRxCT.
Ventral to the fenestra vestibuli on the lateral aspect of the promontorium is a large
groove, measuring 0.40 mm in diameter (Fig. 3.9: g1). This groove is directed
anterodorsally. It is the g1 groove (Fig. 2.40). At its posteriormost point, it is associated
with the posterior septum (MacPhee, 1981), which covers the aperture for the fenestra
cochleae, or “round window,” (Fig. 3.9: ac) and contains the internal carotid canal in its
ventral margin (Fig. 3.9: ps). The anterodorsal continuation of g1 onto the promontorium
appears equivalent to what has been termed g5 for other plesiadapids (Fig. 2.40; Fig. 3.9:
g5). There are other grooves on the lateral aspect of the promontorium. One seems to
run from medial to lateral starting from the lateral side of the anterior apex of the
posterior septum where it arises from the promontorium (Fig. 3.9: tng). As this groove
emerges from the fossula for the fenestra cochleae, formed by the posterior septum its
canal deepens and runs vaguely parallel and laterodorsal to g1. After moving a short
distance anteriorly on the petrosal, it dives under a bridge of bone and thus becomes a
short canal. The canal ends and the groove resumes for a short distance before becoming
shallow and disappearing just anterior and ventral to the anterior apex of the fenestra
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