Mayne commanded HMS "Nassau" on the survey expedition to the Straits of Magellan, 1866–9. The naturalist on the voyage was Robert Oliver Cunningham. Charles Darwin requested the Lords of the Admiralty to ask Capt. Mayne to collect several boatloads of fossil bones of extinct species of quadrupeds. Admiral Sulivan had previously discovered an astonishingly rich accumulation of fossil bones not far from the Straits. These remains apparently belonged to a more ancient period, than the collection by Mr Darwin on HMS "Beagle" and by other naturalists and therefore of great interest to science. Many of these were collected with the aid of Hydrographer Capt. Richards R.N. and deposited in the British Museum.