An 'exceptionally rare' letter sent and signed by world-famous fossil collector Mary Anning discussing her work could fetch up to £12,000 when it is sold at auction.

The 3-page letter, to pioneer palaeontologist William Buckland sent in February, 1829 recounts her discoveries on the west Dorset coast including the skeleton of a plesiosaur.

Although Mary Anning, from Lyme Regis, was something of a celebrity in her day, her gender and class prohibited her from ever becoming a member of the scientific establishment and it is only in recent decades that her importance has been fully acknowledged.

She will soon be brought to life by Kate Winslet in the film Ammonite due to be released this year.

The majority of Anning's papers were discarded as of little value at the turn of the twentieth century, and only a relatively small number of letters survive.

Sotheby's, which is selling her letter to William Buckland in an online auction on Tuesday, says it has no record of any autographed letter by Anning having previously been offered at auction. It is expected to sell for between £8,000-£12,000.

The letter explains that she delayed sending him specimens because the recent frost has affected her work on the cliffs, but she has now sent him a box ("...there are few coprolites which I hope you will think good there is one with bits of Sepia in it another in marle with some remarkable bones in it one has an impression of an Ammonite..."). She passes on messages from the local collector Elizabeth Philpot, then continues with discussion of her "new skeleton" ("...I forget wether I told you it had two hundred & fifty Vertebrae the tail wanting & I have no doubt if it had a tail that it would have more then 3 hundred of them...").

She explains that as Bristol has offered only £30 she will send her drawing of it to Baron Cuvier, despite her misgivings about it going overseas.

The box of specimens that Anning had sent are now, in all likelihood, part of the collection of the Oxford Museum of Natural History.

The "skeleton" referred to was undoubtedly the Plesiosaurus dolichodeirus she discovered on January 29, 1829, which is now in the London Natural History Museum.