Anapsid

An anapsid is an amniote whose skull does not have openings on the sides behind the eyes. This is

1. A primitive ('basal') condition for early amniotes in the Mississippian (early Carboniferous).
2. A secondary ('derived') condition of some other reptile groups, probably including turtles (Chelonia).

General diagram of the anapsid skull.

Some other groups were anapsids, such as the mesosaurs.