Astrophytum asterias is a Chihuahuan desert species of cactus commonly called sand-dollar or sea-urchin cactus.  It is a low growing plant to about 3 or 4 inches in diameter and has no spines and a yellow flower to about 2 inches.  The larger white tufts are the 'areoles' of the cactus.  This is the point where the flowers emerge,  (near the growing tip in the case of this species), and where the spines grow out of on plants that have them.  A. asterias is native to the border region along the Rio Grande from southern Texas into northern Mexico.  The genus astrophytum means 'star-plant' and the species name asterias also refers to its star-like pattern of ribs.