The U.S. Fish and Wildlife Service yesterday designated 4.1 million acres as critical habitat for the threatened California red-legged frog. The designation, the state’s largest habitat area for a threatened species, spans 28 counties from Northern California to Riverside County near Los Angeles. It requires landowners seeking federal building permits to prove that that their projects won’t harm the jumping frog, which was first made famous in a Mark Twain story. The frog has disappeared from 70 percent of its range, with only four regions supporting populations greater than 350.