Argyrochosma incana has been collected at about 10 localities in the flora area, all within 50 km of the Mexican border. Most herbarium specimens were previously identified as A . limitanea , which is easily distinguished from A . incana by the characteristics given in the key. In addition, all known plants of A . limitanea have 32 spores per sporangium; those of A . incana have 64. Although A . incana shows considerable morphologic and biochemical diversity in Mexico, populations in the United States are relatively uniform.