What is singular possessive and plural possessive form of Mavis?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: Mavis'

Add an apostrophe s ('s) after the existing s at the end of the Word: Mavis's

The plural form of the noun Mavis is Mavises.

The plural possessive form is Mavises'.

Examples:

I have Mavis' phone number. (singular)

I have Mavis's phone number. (singular)

The two Mavises' last names are White and Greene. (plural)