The peripheral vestibular membranous ducts that detect angular motion are contained within bony semicircular canals of mammalian petrosals. I investigated morphology and function in the three membranous semicircular ducts through measurements on the bony semicircular canals of 31 skeletonized skulls from different genera. While the prevailing theory of semicircular canal researchers is that the locomotor agility of extant and extinct mammals can be understood by measuring the size of the three bony semicircular canal arcs, I propose that there are important and quantifiable features other than the adult size of radius of curvature of the semicircular canal arc (R) that influence angular movement detection in mammals and perhaps in their ancestors. Initially, I sought to verify that there was no significant asymmetry of R across the study specimens. However, there was significant asymmetry in canal pair angles between contralateral sets: ipsilateral canal pair angles differed by up to 14◦, and contralateral synergistic angle pairs differed by up to 18◦. Canal pair contralateral differences were lower for specimens of more agile taxa. In addition, the angle between the left and right lateral canals varied by up to 27◦ from parallelism, so the use of the lateral bony canal in one petrosal to represent the horizontal animal resting position could result in significant skull orientation errors. I utilized a program to quantify the effects of canal plane non-orthogonality and to calculate a maximum rotational sensitivity axis for a given taxon. My results concur with earlier research indicating that canal orientation significantly affects the location of maximum rotational sensitivity axes in the head, and should be considered in future quantitative research. Finally, I determined the volumes of the subarcuate fossa and the petrosal lobule in three Monodelphis domestica animals (76 days postnatal) by utilizing both cranial and tissue volumes in fresh specimens. The petrosal lobule fills 93-97% of the Monodelphis domestica fossa, a greater volume than the 50% estimated by previous researchers. These results highlight the difficulties of using histologic or preserved specimens to make quantitative determinations of brain tissue volumes, and reopen the question of whether the subarcuate fossa volume provides a record of the agility for an extinct taxon. / text