Series
The plural is uninflected.
Stadia vs. stadiums
Stadiums is far more common in 21st-century usage.
Species
It is both singular and plural.
Summons and summonses
Summons is a singular noun meaning a call by an authority to appear or to do something. It’s most often used in legal contexts for notice summoning a defendant or witness to appear in court. Despite the s at the end, summons functions like any other singular noun—for example: Evansville police said they will see that a court summons is issued for a man who they say was too drunk to go to jail early Sunday. [Evansville Courier & Press] …
Criteria, criterion
Criteria is becoming singular, and criterion is disappearing from English, but we are still early in the process and the more traditional forms remain safer in formal writing.