The turtle/tortoise note is correct for UK English, but in the US (where Id software was headquartered during the development of Quake), tortoises are considered a subset of "turtles". Per Webster:

> Turtle (noun): any of an order (Testudines synonym Chelonia) of terrestrial, freshwater, and marine reptiles that have a toothless horny beak and a shell of bony dermal plates usually covered with horny shields enclosing the trunk and into which the head, limbs, and tail usually may be withdrawn.