Heloderma, the only
genus of the family Helodermatidae, consists of
lizards native to the southwestern
Mexico, and as far south as
Guatemala. It includes five separate
species, one with two
subspecies. Their closest living relatives are the
Helodermatids (or beaded lizards) are large, stocky, slow-moving reptiles that prefer semi
 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
 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.
rodents and other small
mammals, and eating the
birds and reptiles. They are
oviparous, laying large
clutches of eggs.