The shapes and dimensions of the cochlear cavities from four petrosals of the genus Morganucoden obtained through sectioning and reconstruction. Morganucodon dates from the early Jurassic and represents many of the earliest known mammal specimens. Each Morganucodon petrosal fossil was embedded in Araldite and shaved with an ultramicrotome to expose the internal structure. Line drawings of the exposed cross-sections were digitized and used to produce three-dimensional reconstructions. The reconstructed Monganucodon cochlear cavities differ from extant mammalian cochleas in several respects: they are uncoiled, shorter in length, and lack the bony lamina which supports the basilar membrane. These three features ar characteristic of extant Aves and Reptilia.