It can go both ways, but both are only acceptable under different occasions:
To quote "MLA Handbook for Writers of Research Papers," Fourth Edition, 2.2.7:
Quote
b. To form the possessive of a plural noun ending in s , add only an apostrophe.
*** photographers' props
- firefighters' trucks
- tourists' luggage**
BUT, the case that applies to this (off-)topic the most follows:
Quote
e. To form the possessive of any singular proper noun, add an apostrophe and an s.
*** Venus's beauty
- Dickens's reputation
- Descartes's philosophy
- Marx's precepts**
f. To form the possessive of a plural proper noun, add only an apostrophe.
*** the Vanderbilts' estate
- the Jones' economic woes
**
Therefore I rule in favor of Dr Tall. Case dismissed.
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