Have you ever wondered when you may "legally" add a "'s" to denote possessive of a proper noun? According to The Chicago Manual of Style, 15th Edition, the general rule for possessives of nouns covers most proper nouns is formed by adding an apostrophe and an "s." This includes most names ending in sibilants (kind of a hissing or noisy s-z sound).
For names ending in silent s, z, or x the possessive, unlike the plural, can generally be formed in the usual way (use 's) without suggesting an incorrect pronunciation. It uses "Josquin Des Prez's motets" as an example.
But CMS and The Gregg Reference Manual, Seventh Edition, do give you a way out: If the addition of an extra syllable would make a word ending in an "s" difficult to pronounce, add an apostrophe only. Individual differences in pronunciation will affect the way some of these possessives are written....the important thing is to listen to your own pronunciation.
Prentice Hall Reference Guide to Grammar and Usage, Fifth Edition, says the same thing as Gregg, and again refers to either common or proper nouns. They both cite examples where a proper posssive noun is followed by a word beginning with a sibilant, e.g., Euripides's story is difficult, so say Euripides' story.
And read Professor Charles Darling's comments: http://grammar.ccc.commnet.edu/grammar/possessives.htm