Lefty, Lucky, the Ant, Bugsy, the Snake, the Chin, Scarface, the Brain. The monikers of mobsters are like the nicknames of odd superheroes. They are two syllables of rat-tat firing, evoking creepy animals, physical protrusions or uncanny powers. And now, here in Las Vegas where such figures were once as comfortably in their element as Zeus and his family on Olympus, they are finally getting something close to the museum they deserve: the Mob Museum, a $42-million survey of the American gangster, unfolding in 17, 000 square feet of exhibition space, on three floors of a 41, 000-square-foot landmark building on Stewart Avenue. With artifacts, clever interactive displays, atmospheric exhibits and photographs and videos, we learn how Las Vegas developed out of the early-20 th-century desert, and how workers on the nearby Hoover Dam gave the town its first population explosion.