The Beta Portolan system was a formerly inhabited planetary system. This system was home to ancient civilizations that were destroyed by a species of neural parasites. Archaeologists had found evidence that this system was the beginning of the invasion of the Milky Way Galaxy by the neural parasites. The inhabitants of this system transported the parasites to Levinius V. The primary of this system was Beta Portolan. (TOS: "Operation -- Annihilate!"; Star Trek VI: The Undiscovered Country)
This system was only mentioned in dialogue.