weak venom and no fangs. According to this theory, most snakes that are labelled "nonvenomous" would be considered harmless because they either lack a venom delivery method or are incapable of delivering enough to endanger a human. The theory postulates that snakes may have evolved from a common lizard ancestor that was venomous, and also that venomous lizards like the gila monster, beaded lizard, monitor lizards, and the now-extinct mosasaurs, may have derived from this same common ancestor. They share this "venom clade" with various other saurian species. Venomous snakes are classified in two taxonomic families: Elapids – cobras including king cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes. Viperids – vipers, rattlesnakes, copperheads/cottonmouths, and bushmasters. There is a third family containing the opistoglyphous (rear-fanged) snakes (as well as the majority of other snake species): Colubrids – boomslangs, tree snakes, vine snakes, cat snakes, although not all colubrids are venomous. Reproduction Although a wide range of reproductive modes are used by snakes, all employ internal fertilization. This is accomplished by means of paired, forked hemipenes, which are stored, inverted, in the male's tail. The hemipenes are often grooved, hooked, or spined—designed to grip the walls of the female's cloaca. The clitoris of the female snake consists of two structures located between the cloaca and the scent glands. Most species of snakes lay eggs which they abandon shortly after laying. However, a few species (such as the king cobra) construct nests and stay in the vicinity of the hatchlings after incubation. Most pythons coil around their egg-clutches and remain with them until they hatch. A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even "shiver" to generate heat to incubate the eggs. Some species of snake are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch. Several species of snake, such as the boa constrictor and green anaconda, are fully viviparous, nourishing their young through a placenta as well as a yolk sac; this is highly unusual among reptiles, and normally found in requiem sharks or placental mammals. Retention of eggs and live birth are most often associated with colder environments. Sexual selection in snakes is demonstrated by the 3,000 species that each use different tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes