Head with the snout produced, pyramidal, covered with small, scale-like, but not imbricate, shields. Teeth acute, compressed. Tongue elongate, slender, terminating in a long fork, retractile into a sheath at its base. Scales small, equal on the sides and on the back, arranged in cross rings; those of the belly and tail square, in cross bands. Tail long, generally compressed. Toes 5-5, armed with strong claws.
This family contains the largest species of Lizards; the greater part of them live in the neighbourhood of large rivers, and are excellent swimmers, their long, compressed tail serving as a propeller; they are carnivorous, feeding on all the different water-animals and on the eggs of birds, and likewise on those of other large reptiles.
Their movements on land are not much less rapid than in the water. Several species climb trees; they are active during a part of the night. The external nasal opening leads into a spacious cavity situated on the snout; when the animal dives, it closes the nasal aperture, and retaining a certain quantity of atmospheric air in that pouch, or rather in the two pouches, it is enabled to remain under water for a prolonged period without the necessity of rising to the surface in order to breathe. It is the same plan of structure as that with which a large northern Seal (Cystophora borealis) is provided.

They are found in the tropical parts of Africa, Asia, and Australia. The species of British India belong to the following genera:-
Nostrils an oblique slit, in, or nearly in, the middle between eye and extremity of snout .......... Varanus, p. 64.
Nostrils a more or less rounded opening, near the extremity of the snout     Hydrosaurus, p. 67.
* This is referred to by lElian, who says that the Ganges is inhabited by Crocodiles which have a horn on the end of the snout (Hist. Anim. lxii. c. 41).