James Clemens House is crumbling. That's a shame, because this stately landmark in the city's near north side has a rich pedigree. Mark Twain's cousin James Clemens Jr., who was one of the area's first millionaires, built the brick-and-cast-iron structure in 1860 after his wife Eliza died of cholera — which explains why the exterior window lintels are adorned with likenesses of her face. In later years it became a convent, then a homeless shelter. According to Michael Allen, founder of the Preservation Research Office, the mansion's roof is steadily caving in, endangering the ornate parlor rooms on the ground floor. Developer Paul McKee owns the property, and he hasn't done much to slow the decay. Still, Allen remains hopeful: "It's not yet past the point of no return." 1849 Cass Avenue, St. Louis, 63106.