Heloderma, the only genus of the family Helodermatidae, consists of venomous lizards native to the southwestern United States, Mexico, and as far south as Guatemala. It includes five separate species, with two subspecies. Their closest living relatives are the anguid lizards.Helodermatids (or beaded lizards) are large, stocky, slow-moving reptiles that prefer semiarid habitats. Their tails are short and used as fat storage organs.