The most useful rule — and the most general and the easiest to remember — is simply that you add ’s whenever you actually say an extra /əz/ at the end when forming the possessive, compared with how you say the non-possessive version. Let your own ear be your guide. That’s all there is to it. No fancy rules full of exceptions. Just your own ear (as a native speaker, mind you).
So words ending in unstressed /iːz/ are exempt, like for example this series’ end, that species’ demise, Mercedes’, Ramses’, Sophocles’, Socrates’, Achilles’, Diomedes’, Archimedes’, Eratosthenes’, Ulysses’. (But not trapeze’s, because that one is stressed! See how that works?)
But these days, not much else is. I say “in these days” because in previous ages, some people did not add another /əz/ if it already had one, and so wrote Jesus’ to indicate they did not say an extra /əz/ there compared with Jesus: both are just /ˈd͡ʒiːzəs/ However, most people today now say Jesus’s, because it has three syllables: /ˈd͡ʒiːzəsəz/.
Same with Moses’s with three syllables instead of the older Moses’ with just two. Note that things like Ross’ and Chaz’ are always wrong, because no one says those with only a single syllable. That is a common error.
So it’s your boss’s house, because it’s got an extra syllable when you say it. Similarly, all the Jameses I have ever personally known have had the extra /əz/ tacked on when you are talking about something of theirs, which means it is for those speakers James’s house, albeit all the Jameses’ houses, because nouns are only allowed one /əz/ inflection, not two.
In all cases, the best thing to do is let your own ear be your guide, because writing should represent speech. That means that if you say an extra /əz/ then you write ’s, but if you don’t say it, then you don’t write it. That’s why you from time to time see forms like for goodness’ sake or for conscience’ sake. Those are possessive, but have no extra syllable.
As for the specific case of Aeneas, in older writing you will find that because his name already ends in /əz/, people would suppress the extra one when they would form the possessive, like Aeneas’ escape from Troy. Note very carefully that that posits a three-syllable possessive when spoken. If when you yourself say it, however, it turns out that you would yourself use the four-syllable version as most people today now do, then it would have to be Aeneas’s escape from Troy.
But now you have three issississes in a row, which will certainly require careful elocution to pull off — especially if you don’t mean to sound like Gollum with his fisheses.
The online Chicago Manual of Style (both 16th and 17th editions) states:
When the singular form of a noun ending in s is the same as the plural (i.e., the plural is uninflected), the possessives of both are formed by the addition of an apostrophe only. If ambiguity threatens, use of to avoid the possessive.
And gives the following examples:
politics’ true meaning
economics’ forerunners
this species’ first record (or, better, the first record of this species)
This is section 7.19 of the 16th edition and 7.20 of the 17th edition (2017).
Obviously this differs to my comments saying that the CMOS says species's, which I took on faith from a third party website.
However, in speaking, despite remarks by others, I'm not sure I would rule out saying the species's survival (with the extra syllable) just to clarify that I'm talking about the possessive form of the word. This is my opinion only.
Best Answer
The style ruling you cite is not entirely consistent with regard to Massachusetts', New Orleans', and Paris's. That's not to say that it's wrong, but it does seem to be arbitrary—and it is by no means universally accepted by other publishing style guides.
For instance, here is a different guideline from Words Into Type, Third Edition (1974):
Because the last syllable of Massachusetts contains another sibilant besides the final s, the WIT guideline would render its possessive as Massachusetts'; WIT also agrees with the Merriam-Webster guideline on Paris's (assuming that we're talking about the French city and not the Trojan Helen-napper). But the WIT rule would seem to endorse using New Orleans's as the possessive of New Orleans.
The Chicago Manual of Style, Fifteenth Edition (2003) likewise has a lengthy, detailed, and ultimately arbitrary set of guidelines on this point:
In the Chicago system, the possessive forms of the three words that Merriam-Webster's uses as examples would be Massachusetts's, Paris's, and New Orleans's. I should note, however, that after delineating its system in great detail, Chicago blandly says "Those uncomfortable with the rules, exceptions, and options outlined above may prefer the system, formerly more common, of simply omitting the possessive s on all words ending in s." That would yield Massachusetts', Paris', and New Orleans'.
The Associated Press Stylebook and Libel Manual (2002) adopts the system that Chicago mentions at the end of its coverage:
For sheer consistency, you can't beat AP's guideline. Chicago and Words Into Type provide generally consistent guidelines and try to spell out rules for recognizing exceptions to the general rule. Merriam-Webster's Guide to Punctuation and Style doesn't identify the basis for exceptions to the general rule represented by "Paris's cafes"; it simply observes that "The possessive of ... some singular proper names ending in an s or z sound, is made by adding just an apostrophe." So should you write the possessive of Kansas as Kansas's or Kansas'? The MWGPS guidelines don't say, whereas the guidelines provided by WIT and Chicago do—though they lead to different conclusions.
I don't know how Merriam-Webster's Guide to Punctuation and Style came up with the advice it offers on Paris's, Massachusetts', and New Orleans', but the book doesn't seem to provide much predictive guidance for forming possessives of other proper names ending in s.