Spoilers for Monday's Supergirl episode.
Monday's episode of Supergirl, entitled "Solitude," featured numerous Easter eggs from DC Comics continuity as Kara visited her cousin's Fortress of Solitude. Among the various artifacts on display was an iconic flight ring from the Legion of Super-Heroes.
Teenage heroes from the 31st century, in comic books, the Legion traveled back in time to meet their hero, Superboy, who wound up having adventures with the team for years. In later comic books, Supergirl also adventured with the Legion.
And this isn't the first time the Legion have been hinted at in DC TV -- another ring was spotted during the Flash's visit to the Bleed in The Flash's "Welcome to Earth-2" episode. And the Legion appeared in the flesh during the CW's Smallville.
Also seen in Monday's Supergirl was the Omegahedron, a device featured in the 1980's Supergirl film starring Helen Slater who plays Kara's adoptive mom on CBS's Supergirl. In the movie, the Omegahedron was a power source for Kara's home of Argo City.
Supergirl airs Monday nights on CBS.