Sibilants at the ends of words

From ProofreadNOW:

A sibilant is the hissing sound in speech that’s produced with s, sh, z, zh, or x. (e.g. sash)

The general rule for the possessive of nouns covers most proper nouns, including most names ending in sibilants:

• Kansas’s corn fields
• Robert Burns’s poems
• Karl Marx’s theories
• June Jones’s great throwing arm
• Jefferson Davis’s Beauvoir
• Texas’s Baylor University

For names ending in silent s, z, or x the possessive, unlike the plural, can generally be formed in the usual way without suggesting an incorrect pronunciation:

• Margaux’s bouquet
• Descartes’s works
• Josquin Des Prez’s motets

Traditional exceptions to the general rule for forming the possessive are the names Jesus and Moses:

• in Jesus’ name
• Moses’ leadership

Names of more than one syllable with an unaccented ending pronounced eez form another category of exceptions. Many Greek and hellenized names fit this pattern. For reasons of euphony the possessive s is seldom added to such names:

• Euripides’ plays
• Xerxes’ army
• Ramses’ tomb

Pluralizing. Names of persons and other proper nouns form the plural in the usual way, by adding s or es:

• all the Edwards and Charleses
• two Walden Ponds
• three Marys

Note that the apostrophe is never used to denote the plural of a personal name: “The Drews left Florida today” (not “The Drew’s . . .”).

Source: Chicago Manual of Style