Edit:
We overlapped, possibly because I didn't refresh, so to answer; the apostrophe thing is a matter of style (and occasional debate), in real speech, people often say them both as if written "Atlas's" i.e. identical to the plural "Atlases" if that's easy enough to do. Pedants would usually argue that what you should do is "-'s" for the singular possessive and pronounce it and apostrophe only and not pronounce any ess for the plural possessive.
In short, clarity is what people strive for and this is how and why rules simplify and foreign words end up being treated just like any other.
Edited by SakuranoSenshi, 04 August 2012 - 09:32 PM.