What is "amphitheriida"

Amphitheriida is an order of mesozoic mammals restricted to the Middle Jurassic of Britain. They were closely related to the Dryolestids but possessed five molars instead of the usual four in Dryolestida, (with the exception of the family Dryolestidae whose members possessed between 8 and 9 molars). The Amphitheriida contains one family, the Amphitheriidae.