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: cactus'
Add an apostrophe s ('s) after the existing s at the end of the word: cactus's
Examples:
The night blooming cactus' name is Queen of the Night.
The night blooming cactus's name is Queen of the Night.
The possessive form for the plural noun cacti is cacti's.
The plural forms for cactus are cacti or cactuses, both are accepted.
The plural possessive forms are cacti's or cactuses'.
The noun cactus is the singular.
The plural forms are cacti or cactuses, both forms are accepted.
The form cacti's is the possessive form of the pluralnoun cacti.
The singular noun is cactus. The singular possessive form is cactus's.
Cactus - singular
Cacti - plural
cacti
Cacti is plural form for the singular cactus.
The possessive form is Amos's.
The singular form is goose; the singular possessive form is goose's.The plural form is geese; the plural possessive form is geese's.
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
The singular possessive form is week's.
The singular possessive form is contralto's.
The singular possessive form is aircraft's.
The singular possessive form is frontiersman's.
The singular possessive form is speculum's.
The singular possessive form is herd's.
The singular possessive form is heart's; the plural possessive form is hearts'.
The singular form for the noun families is family. The singular possessive form is family's.