Monitor, the common name of many of the old-world slender-tongued lizards of the family varanidai and genus varanus (Merr.). They have an elongated head; long, extensile, bifid, fleshy tongue, enclosed in a sheath at the base; no teeth on the palate, those of the jaws flattened at the roots, lodged in a common groove or alveolus without internal border, with the crowns generally pointed and curved backward; the neck long; the head and body covered with tuberculated non-imbricated scales; the tail very long, sometimes containing 80 vertebras, capable of reproduction, non-prehensile, compressed and keeled or rounded according as the species are aquatic or terrestrial; no femoral pores nor dorsal crest; eyes with two distinct movable lids; feet large, with five unequal, non-palmated toes, furnished with strong claws; in the fore limbs the first finger is the shortest, and the third and fourth longest; in the posterior the fourth is three times as long as the first. The monitors form a natural transition to the serpents, in the suspension of the bones of the face to the cranium and their mobility, in the incomplete circle of the orbits, in the long and narrow lower jaw loosely united in the middle, in the tongue, and in the scaly covering.
The colors vary from black to deep green, with lighter spots arranged in various ways so as to resemble mosaic work; many of these patterns are so admirable that the skin has been used to cover jewel boxes. These reptiles are, next to the crocodiles, the largest of living saurians; they live either in the neighborhood of rivers, or in dry sandy regions, the former class being said to give notice of the presence of crocodiles by a whistling sound, whence their common name; they run rapidly on the ground, in a serpentlike manner on account of the length of the tail. Their food consists principally of the larger coleopterous and orthopterous insects; they also eat the eggs of aquatic birds and reptiles, and lizards, small tortoises, fish, and mammals. The true monitors, of which fewer than 20 species are described, are confined to Asia, Africa, and Australasia. Of the genus varaius, erroneously called tupinambis by Daudin, the best known aquatic species is the monitor of the Nile (V. Niloticus, Fitz.), common in the rivers of Egypt and of western and southern Africa, and attaining a length of 5 or 6 ft., of which the head is about one eleventh, the neck one ninth, and the tail nearly one half; the teeth are 30 above and 22 below; the general color above is greenish gray with black dots, with four or five yellow V-shaped marks pointing backward upon the nape, bands of yellow eye-like spots on the back, a wide black band on the shoulder, and a narrow one edged with pale green on each temple; whitish below, with brown transverse bands, and the claws black.
From its supposed usefulness in devouring the eggs of the crocodile, it was highly esteemed by the ancient Egyptians. Other aquatic species are found in the East Indies, and in Australia and its archipelago. Of the terrestrial monitors the best known is the V. scincus (Merr.), the skink of the ancients, the land crocodile of Herodotus, the waran of the Arabs, and the genus psammosauras of Fitzinger. This is very common in the sandy deserts of Egypt; it is about 3 ft. long, of which the rounded tail is more than half. The color of the upper parts varies from brown to yellow, spotted and banded with one or the other; it is less carnivorous and ferocious than the aquatic monitors. - Cnvier, in his Ossemens fossiles, has referred to the family of monitors several gigantic fossil reptiles, as the proto-rosaarus (H. de Meyer), from the coppery schists of Germany; the mosasaurvs (Conyb.), over 30 ft. long, intermediate between monitors and iguanas, from the calcareous strata of Maestricht; the geosaurus (Cuv.), 12 or 13 ft. long, from an iron mine near Mannheim; and the megalosaurus (Buckland), about 40 ft. long, from the vicinity of Oxford, placed by Pictet among the dinosaurians, having certain mammalian characters. - The name of monitor is sometimes given to some American lacertian lizards, especially of the genus sahator (Dum. and Bibr.), more properly called safeguards, corresponding in part to tupinambis (Daud.) and tejus (Merr.), and to monitor (Fitz.).
Nilotic Monitor (Varanus Niloticus).
Monitor, in naval architecture. See Ironclad Ships.