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:
cross'
Add an apostrophe s ('s) after the existing s at the end of the word:
cross's
Examples:
The cross' damage was repaired in time for the ceremony.
The cross's damage was repaired in time for the ceremony.
The singular noun is cross; the singular possessive is cross's.The plural noun is crosses; the plural possessive is crosses'.
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
The singular form is comet, the plural form is comets, the possessive plural is comets'. Example: The comets' paths will not cross.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive pronouns are mine, yours, his, hers, and its. The singular possessive adjectives are my, your, his, her, and its.
The singular possessive of dish is dish's
The singular possessive is Martin's.
The singular possessive is fish's.
Grave's is the singular possessive.
The singular possessive is test's.
Buzz's is the singular possessive.
The singular possessive is biker's; the plural possessive is bikers'.