Screen Gems is developing bestselling book series “The Mortal Instruments” into a possible film franchise, starting with the series’ first installment City of Bones. Screen Gems will team with Constantin Film and Unique Features on the project. The “Mortal” series is by author Cassandra Clare. “City of Bones” tells the story of a seemingly ordinary teenager who discovers she is the descendant of a demon-hunting clan called Shadowhunters. The pic is penned by Jessica Postigo.