The femur bone of a dodo and the giant partially fossilised egg of an elephant bird are being put up for auction at Christie's.
The bone, which is expected to fetch between £10,000 and £15,000, is believed to be the first to come to auction since 1934.
It was almost certainly excavated in 1865 at Mare aux Songes, in southeast Mauritius by natural history enthusiast George Clark.
The dodo was wiped out hundreds of years ago after people began populating Mauritius.
James Hyslop, the head of travel, science and natural history at Christie's South Kensington, in London, said: "As an icon of extinction, the dodo is second to none.
"This exciting discovery is one of the few pieces of dodo material in private hands, and it is a privilege and humbling experience to have been entrusted with the bone.
"It is a reminder of the effect humans have on the natural world, and presents a rare opportunity to engage with this now lost and most enigmatic bird."
The enormous elephant bird egg is 30cm high and 21cm in diameter - more than 100 times the size of a chicken egg.
The elephant bird, which resembled an 11-foot-tall ostrich, is thought to have been hunted to extinction in Madagascar between the 14th and 17th centuries.
Examples of whole eggs are extremely rare, and the egg on sale is likely to fetch up to £30,000.
More than 260 lots will go under the hammer in the Travel, Science and History sale, including curiosities of natural history, globes, scientific instruments, rare books and maps, and paintings and works of art from the ages of exploration.
Falling into the science category is an Enigma cypher machine, which is expected to go for between £40,000 and £60,000.
Used during World War Two by the German military to encrypt and decode messages, the machine was designed with a complex, interchangeable series of three rotors bearing the 26-character alphabet, a "reflector" and a plug board with movable cords that connected pairs of letters.
With 15 billion possible readings for each character, it was considered too complex to be broken.
But due to the efforts of Alan Turing and fellow cryptanalysts at Bletchley Park, the coding mechanism was solved, enabling the Allies to read all secure messages.
Another German cypher machine, the Schluesselgeraet 41 or SG-41Z, will also be for sale.