Input (grade 13): Most also have laterally compressed bodies, ventral scales are much reduced in size, their nostrils are located dorsally (no internasal scales) and give birth to live young (ovoviviparous).
Output (grade 6): Ventral scales are much reduced in size, the nostrils of the snakes are located dorsally (no internasal scales). They give birth to live young (ovoviviparous).