_The Oxford Companion to the English Language_ has a long article on the
apostrophe.
<<With names of classical origin, a second "s" is not usually added,
especially when the end sound of a word is /z/ rather than /s/:
"Xerxes' battle," "Socrates' pupils.">>
"Descartes" is not of classical origin.
While we are discussing style, I must say that I very much cherish the
American English rules for quotation marks. Please observe tradition and
don't use logical quotation if you write American English.
-- Eivind Berge eivber@online.no