For example: The Blackened Wastes (Tropical Wilderness Desert, 25% Dinosaur, 35% Prehistoric); The Woolly Tundra (Subarctic Wilderness Plains, 75% Prehistoric); The Slithering Jungles (Tropical Civilized Jungle, 60% Dinosaur)
% Dinosaur and % Prehistoric refers to the percentage of random encounters (and presumably placed lairs) that consist of Dinosaurs (including pterodactyls, plesiosaurs, dimetrodons and the like) or "Prehistoric" encounters (such as mammoths, smilodons, cave men, axe beaks, hobbits, etc.) in the noted geographic area, with a roll being made on the appropriate tables for the composition of said encounter, such as the tables in the 1st Edition Dungeon Masters Guide.
This idea came up while brainstorming comprehensive modular encounter table methodology. Obviously other %s suggest themselves, such as % Robot, % Snake & % Supervillain.
EDIT: Of course I'm an idiot for not stating the obvious % Lovecraft...