"Eothyris" is known only from its complete skull, however, the postcranial skeleton is unknown. Its skull is short and broad with a total skull length of , Benson "et al." estimated the body to be long. The major distinguishing features of the skull are that it possessed a pair of long, large, fang-like teeth on each side of the upper jaw. The main differences between "Eothyris" and "Oedaleops" are related to the degree of specialization in the dentition of the geologically younger "Eothyris".