Possessive plurals are odd with irregular nouns...elk, deer, sheep. I always thought if you moved the apostrophe it went from singular possessive to plural possessive....

deer's  ....deers'  elk's....elks'  but today the English teacher said no....the only way is deer's or elk's  or sheeps'.  but she said plural possessive of these words that singular and plural are the same, you can only add 's not s' because you just take the plural as it is and make it possessive.  So some sentences you would not be able to differenciate whether it referred to one or many of that animal.  The sheep's feet are sore.  one sheep or 100?