The weekend’s new reboot of the “X-Men” series may have shapeshifting, magnetism and ambient energy conversion — but the thing it needs now is “legs.”

“X-Men: First Class” won the weekend with a domestic-opening gross of $56-million, according to studio estimates Sunday. That’s the lowest opening for an “X-Men” movie since the first film in the series debuted at $54.5-million — but that was in year-2000 dollars (that opening would be roughly $80-mill in adjusted-for-inflation 2011 bucks). Final weekend figures are expected to be released Monday.

By comparison, 2006’s “X-Men: The Last Stand” grossed $105.8-million when it opened on a Memorial Day weekend. 2009’s “X-Men Origins: Wolverine,” opened with $85.1 million.

On the plus side for Matthew Vaughn’s “First Class” is that word-of-mouth should continue to spread about how the younger talents fare in this reboot to the ‘60s — particularly stars James McAvoy (taking over the Professor Xavier role from Patrick Stewart) and the white-hot Michael Fassbender as Magneto, as well as Oscar nominee Jennifer Lawrence (Mystique).

In her praise of the film, Post critic Ann Hornaday wrote: “ ‘First Class’ belongs to one actor, and that’s Fassbender, whose Erik/Magneto emerges as one of the most nuanced, conflicted, genuinely antiheroic protagonists in recent comic-book-movie memory.”

Working against “First Class,” however, is that big-budget competitors in the weeks ahead are stacking up like Next Generation 737s over O’Hare.

“Super 8” opens Friday and DC’s “Green Lantern” drops June 17 — followed in short order by “Cars 2,” the latest “Transformers” and next month’s “Captain America: The First Avenger.”

Marvel’s first summer-of-’11 film, “Thor,” made $4.2-million to finish sixth. Kenneth Branagh’s tale of Asgard has now grossed $169-million domestically and $423-million globally.

Elsewhere, sequels held strong: “The Hangover Part II” was second with $32.4-million, “Kung Fu Panda Part 2” ($24.3-million) was third and “Pirates of the Caribbean: On Stranger Tides” ($18-mill) was fourth.

This post previously appeared on the Comic Riffs blog.

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