I've read (in the Elements of Style) that, while genitives of names ending in ‘s’ may have an additional ‘s’, as in "Ross’s", this oughtn't to be done with ancient names:
Exceptions are the possessives of ancient proper names in -es and -is, the possessive Jesus', and such forms as for conscience' sake, for righteousness' sake. But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced by
the heel of Achilles
the laws of Moses
the temple of Isis
What is known about the origin of and reasoning behind this distinction?