The Gila monster chews to deliver its venom, as does the BTS and (some) other colubribs. Now I have read that venom has evolved from three different sources (I think that Rick Shine says this but I could be wrong). Now it is possible that the delivery Apparatus has evolved three times, in colubribs, vipers and elapids. That does make some sense, we have the weapon, how to develop the armory.