"-ense" is the neuter ending. It is used following genus names that are neuter nouns (typically ending in "-um" - Cerithium, Cymatium, Phalium, Vasum, etc.). "-ensis" is the ending if the genus name is either feminine (typically ending in "-a" - Oliva, Cypraea, Cancellaria, Terebra) OR masculine (typically ending in "-us" - Conus, Cantharus, Fusinus, Strombus). There are other, less frequent feminine endings besides "-a" and other masculine endings besides "-us". "-ensis" would also be used in those cases. Paul M.