The morphology of the external surface of the petrosal and its junction with the basioccipital in Paramys copei is described. Comparison with an outgroup of the Cretaceous Vincelestes and Recent Didelphis and Solenodon reveals that the auditory region retains many primitive features. The Eocene paramyines from North America show only slight differences. Sciuravus is set apart by the lack of a ventral petrosal sinus canal between the petrosal and basioccipital and by the facial nerve and stapedial artery sharing a common foramen in the petrosal. European theridomyids, too, are not as primitive as Paramys but share with it and Sciuravus a ridge on the promontorium that separates the transpromontorial continuation of the internal carotid artery from the origin of the tensor tympani muscle. Twelve characters of the auditory region are analyzed and summarized in a data matrix for use in future studies. Relative primitiveness of the auditory region in all of these Eocene rodents suggests that derived characters in later rodent taxa may represent intraordinal relationships but only homoplasy with nonrodents.