"Also, at night, crossing the bright disks of Gor's three moon, might ocassionally be seen the silent, predatory shadow of the ul, a giant pterodactyl ranging far from its native swamps in the delta of the Vosk."

Book 2, Outlaw of Gor, page 26

"Only one creature in the marshes dares to outline itself against the sky, the predatory Ul, the winged thalarion."

Book 6, Raiders of Gor, page 1

"The delta of the Vosk, for most practical purposes, a vast marsh, an area of thousands of square pasangs, where the Vosk washes down to the sea, is closed to shipping. It is trackless and treacherous, and the habitat of marsh tharlarion and the predatory Ul, a winged lizard with wing-spans of several feet."

Book 13, Explorers of Gor, page 26 ~¤~