Along with scale taxonomic schemes and even their organs are slightly different in form and shape.

Encyclopedia definition.
The true boas are divided into two subfamilies, Boinae and Erycinae. Boinae includes the boa constrictor (Boa constrictor), tree boas (genus Corallus), and anacondas (genus Eunectes) of the American tropics; two other genera are found on Madagascar and islands of the southwestern Pacific. Members of Boinae range from 1 metre (3.3 feet) long in some species to commonly more than 4 metres in the giant, or green, anaconda. The boa constrictor occupies a variety of habitats from coastal northern Mexico and the Lesser Antilles to Argentina; though seldom more than 3.3 metres (11 feet) long, some have grown to more than 5 metres. Several tree boas possess sizable teeth used for catching birds. An example is the 1.8-metre (6-foot) emerald tree boa (Corallus caninus) of tropical South America; the adult is green above, with a white dorsal stripe and crossbars, and yellow below. The rainbow boa (Epicrates cenchria) of Costa Rica to Argentina is not strongly patterned but is markedly iridescent. Except for the anacondas, most boines are terrestrial to strongly arboreal. The young often move from the trees to the ground as they get older and larger. Most species have labial (lip) pits with heat-sensing organs that complement their sense of smell and excellent vision. Mammals and birds are common prey, which is usually captured by a bite-grasp followed by constriction.

Subfamily Erycinae includes 10 Asian, Indian, and African species of sand boa (genus Eryx) and the West African earth python (Charina reinhardtii), in addition to two North American species. Erycines are live-bearers (as opposed to egg layers) that have stout cylindrical bodies, blunt heads, and short tails. Most measure less than 70 cm (28 inches). These terrestrial snakes are often subterranean, and most live in arid and semiarid habitats, where they prey on lizards and small mammals. The brown, 45-cm (18-inch) rubber boa (Charina bottae) of western North America is the most northerly boa and is a burrower that looks and feels rubbery. The 90-cm (35-inch) rosy boa (Charina trivirgata), ranging from southern California and Arizona into Mexico, usually is brown- or pink-striped
From Britannica