The Omission of S in the Possessive Case.
It is not uncommon with some persons to omit the s after the apostrophe in the possessive case of nouns, if the name itself ends in s; as, “James’ book,” “Barnes’ Notes.” But this is incorrect; for if we ask, Whose book? we should directly answer, James’s. The only case when the s can be judiciously omitted, and this solely to avoid the too hissing sound of so many s’s in succession, is when the first word ends with the sound of s in its last two syllables, and the next word begins with s; as in Misses’ spectacles, righteousness’ sake, conscience’ sake.