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			Just checked the Oxford style guide on this and it says: 
 
'Use an apostrophe alone after classical or classicizing namres ending in s or es: Herodotus', Demosthenes', etc.' 
 
'Use 's after French names ending in silent s or x, when used possessively in English: Dumas's, Descartes's', etc.' 
 
'An apostrophe alone is also permissible after longer non-classical or non-classicizing names that are not accented on the last or penultimate syllable: Nicholas'(s), etc. Jesus's is acceptable in non-liturgical use. Jesus' is an accepted archaism.' 
 
'Use 's after non-classical or non-classicizing personal names ending in an s or z sound: Charles's, Marx's, Tobias's, etc.'			 |   
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