Strunk & White's The Elements of Style concur with 's for possessive. An
exception, they say, is ancient proper names (e.g., Moses, Jesus,
Sokrates), when you use the apostrophe alone w/o following s. There are a
few writers named Xanthus (Gk. chestnut, auburn, Blondie!), but I suppose
they are a whole 'nother story.