All 5 lizards are venomous. Their venom glands are located at the end of their lower jaws, unlike snakes' venom glands, which are located behind their eyes. Also, unlike snakes, beaded lizards lack the musculature to inject venom immediately. The lizards have to chew the venom into the flesh of a victim. Heloderma- venom is used only in defense. 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".