Apostrophes in plurals of nouns ending in s.
While looking in Google Books for early use of the spelling fetus (as opposed to foetus) in British books, I came across this, from the Transactions of the Royal Society, London, with its double use of apostrophes in plurals ending in s - species's and fetus's:
I had known that one of the early uses of the apostrophe was in plurals of certain words ending in vowels (see next section), but this one was new to me.