The noun 'series' is an uncountable (mass) noun, a type of noun called an aggregate noun (a word representing an indefinite number of elements or parts), with no singular form.
The possessive form of the noun series is series'.
example: This book is the last one, the series' end.
Series is a noun that has the same form in the singular and the plural. There is no need for an apostrophe, unless it is in the possessive case: series's for the singular possessive and series' for the plural possessive.
The singular possessive form of "test" is "test's".
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 is ant's.The plural possessive is ants'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive of dish is dish's
Taco's is the singular possessive.
The singular possessive form is classmate's.
The singular possessive is biker's; the plural possessive is bikers'.
The singular possessive is fish's.
Grave's is the singular possessive.
The singular possessive is Martin's.