Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
SPECIAL SITUATIONS
Even armed with the above rules, writers often feel they are on
shaky ground in certain circumstances. In some cases there are
genuine exceptions to the rules; in others, people mistakenly think
that something is an exception, and so fail to follow a rule when it
should apply. These problem areas are described below.
SIBILANTS
Writers are often unsure how to deal with the possessives of nouns
ending in s or another sibilant sound ( ce, x, z), or in a silent s. The
answer is, in most cases treat such words exactly as you would any
other. Thus, to form the possessive of the singular, add apostrophe
s; to form the possessive of the plural, add es to the singular to
make it a plural, and then add an apostrophe.
Ms. jones's property
Mr. Harris's store
the actress's contract
the box's contents
the quiz's answers
the joneses' property
the Harrises' store
the actresses' contracts
the boxes' contents
the quizzes' answers
However, if it would sound awkward to have two s's next to each
other (one belonging to the word itself and another added to make
the word possessive), you may consider dropping the second one.
A sensible way of deciding which way to go is to say the word aloud,
spontaneously. If it feels natural to pronounce the seconds, include
it; otherwise, drop it. Many authorities would consider either of the
following acceptable:
Dickens's novels
Lloyd Bridges's films
Mr. Williams's dog
the Riverses' car
Dickens' novels
Lloyd Bridges' films
Mr. Williams' dog
the Rivers' car
If you're not sure which way to go, it's usually best to include
the extras, since technically it is correct. There is really no excuse for
creating constructions such as Charles' wife or the Ferris' house.
In a few cases (very few, mind you), convention dictates that the
possessive s must be dropped.
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