Dorsal view of three digital cranial endocasts of mammals illustrating the relative size of the olfactory bulb casts. A. Monodelphis domestica (gray short-tailed opossum); olfactory bulb casts compose 8% of the endocast volume. B. Vombatus ursinus (common wombat); olfactory bulb casts compose 3% of the endocast volume. C. Ornithorhynchus anatinus (duckbill platypus); olfactory bulb casts compose 1% of the endocast volume. Yellow arrows indicate positions of the olfactory bulb casts and white lines are scale bars. Each scale bar = 1 cm. Relative to total endocast volume, the opossum has the largest olfactory bulb casts and the semi-aquatic platypus has the smallest. Data from Macrini (2006).