gila monster (hēˈlə) [key], venomous lizard, Heloderma suspectum, found in the deserts of the SW United States and NW Mexico. It averages 18 in. (45 cm) in length, with a large head, stout body, thick tail that acts as a food reservoir, and short legs with strong claws. Its skin is covered with beadlike scales. Its coloring is marbled, a combination of brown or black with orange, pink, yellow, or dull white. The lizard's movements are slow and clumsy. It feeds on young birds and mammals and on eggs. Because the neurotoxic venom is produced by glands in the lower jaw and the grooved teeth through which it passes are set far back in the mouth, venom does not always enter the wound when a victim is bitten. The gila monster must fix its teeth deeply in a certain position to give a fatal bite. The only other member of the genus Heloderma, the beaded lizard, H. horridum, is a somewhat larger black and yellow lizard, found in W Mexico. These two species are the only known lizards whose venom is fatal. They are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Helodermatidae.
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