text: Red C: the joint between the prefrontal and maxilla.  It allows the maxilla to pivot in the plane of the photograph, and while it does not increase gape, it does facilitate the complex action by which the snake draws prey into its mouth.
  Green A: the joint between the frontal bone and nasal bone. It allows the nose to upturn slightly, increasing gape and assisting in swallowing.
  Green B: allows the lower jaws to bow outwards, further increasing the gape.
  Blue: the joint between the supratemporal and parietal.  Immobile, except for Dasypeltis.
  Snake dentition
  In most snakes, teeth are located on the dentary of the lower jaw, the maxilla, the palatine bone and the lateral pterygoid plate. The latter form an "inner row" of teeth that can move separately from the rest of the jaws and are used to help "walk" the jaws over prey.  Several snake lineages have evolved venom which is typically delivered by specialized teeth called fangs located on the maxilla.
  Most snakes can be placed into one of four groups, based on their teeth, which correlate strongly with venom and lineage.
  Aglyph
  Aglyphous snakes (lacking grooves) have no specialized teeth; each tooth is similar in shape and often size.  When teeth vary in size, as in some bird eaters, they do not vary in shape.  Most aglyphous snakes are non-venomous; some, like Thamnophis, are considered mildly venomous. The feature is not a synapomorphy.