Here's a breakdown:
* Closed postorbital bar: This is a bony structure that completely encloses the eye socket. It's found in many mammals, including most primates.
* Open postorbital bar: This is a less complete structure where the eye socket is only partially enclosed by bone. It's a defining characteristic of prosimians, a group of primates that includes lemurs, lorises, and tarsiers.
Therefore, primates can be divided into two groups based on the presence or absence of a closed postorbital bar:
* Strepsirrhines (prosimians): Have an open postorbital bar.
* Haplorhines: Have a closed postorbital bar. This group includes monkeys, apes, and tarsiers (despite tarsiers being classified as prosimians, they have evolved to possess a closed postorbital bar).
So, while most primates have a closed postorbital bar, there are some important exceptions.