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 two separate species, with six 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. They are covered with small, nonoverlapping, bead-like scales, with osteoderms on the undersides of their bodies. Both species are dark in color, with yellowish or pinkish markings.Members of the family are venomous. Venom glands are located in their lower jaws, unlike snakes' venom glands, which are located in their upper jaws. Also, unlike snakes, helodermatids lack the musculature to inject venom. The venom is typically used only in defense, rather than in subduing prey, and the lizard must chew on its victim to work the venom into the flesh. Venom glands are believed to have evolved early in the lineage leading to the modern helodermatids, as their presence is indicated even in the 65-million-year-old fossil genus Paraderma. Venom production among lizards was long thought to be unique to this genus, but researchers studying venom production have proposed many others also produce some venom, all placed in the clade Toxicofera, which includes all snakes and 13 other families of lizards. However, except for snakes, helodermatids, and possibly varanids, envenomation is not considered medically significant for humans.Helodermatids are carnivorous, preying on rodents and other small mammals, and eating the eggs of birds and reptiles. They are oviparous, laying large clutches of eggs.