The Mexican beaded lizard (Heloderma horridum) is the most famous of the four species of venomous beaded lizards found principally in Mexico and southern Guatemala. It and its congener the Gila monster (Heloderma suspectum) are the only lizards known to have evolved an overt venom delivery system. The beaded lizard is larger than the Gila monster, but has duller coloration, black with yellowish bands of differing width depending on the species.