Several features of the monitor lizards are not found in other lizards but are found in snakes. For example, the monitor lizards are the only lizards to swallow their prey whole, or if it is too big, pieces of it. Other lizards chew and crush their food before eating. Another snake-like characteristic unknown in snakes. These clues indicate that monitors and snakes probably descended from the same ancestor way back in prehistoric time.
Other lizard families also have snake-like features and the burrowing skinks have long, streamlined bodies with reduced limbs. One lizard often mistaken for a snake is the Slow-worm. You can tell the Slow-worm is a legless lizard, however, by the several rows of scales along its belly (snakes have only one row), by the presence of eyelids (snakes have none), and by its notched tongue (snakes have forked tongues).