True Boas, Sandboas (family Erycidae)
Boas are a group of nonvenomous, constricting snakes (family Boidae), most of which are found in tropical America and in Madagascar. Boas bear live young, and in this way they differ from the Old World pythons, which lay eggs. Boas are of ancient derivation, retaining some of the features of their lizard-like ancestors, such as paired lungs (modern snakes have only one), tiny remnants of hind limbs (often called spurs), and a characteristic bone in their lower jaw, the coronoid, which is not found in advanced snakes. Boas have no poison fangs, and they kill their prey by squeezing, though their prey is often bitten first.
Boas, pythons, and wood snakes are classified in the family Boidae, whose members have small nostrils and small eyes with elliptical, vertical pupils. Boas are among the most ancient groups of living snakes, having been present in the Cretaceous period, 200 million years ago, when dinosaurs still stalked the earth. Although boas made up a major part of the snake fauna in the past, today only about 40 species of boas are known. Two quite distinct subfamilies are recognized, the true boas, which are mainly tropical rainforest animals from South America and Madagascar, and the sandboas from the deserts and other arid regions of the northern hemisphere. The reticulated python (Reticulatus) of Asia grows up to 33 ft (11 m) and is one the world's largest snakes.