Euphorbia albomarginata (formerly Chamaesyce albomarginata), whitemargin sandmat or rattlesnake weed, is a small low-growing annual, in the spurge family (Euphorbia, Euphorbiaceae) native to desert, chaparral, and grassland habitats of southwestern North America, from southern and central California to Northern Mexico and Louisiana. It can be easily identified by its small size, dusty green leaves, very flattened growth pattern, and the white circular margin around the edge of its burgundy centered flowers. It is the only member of the former Chamaesyce genus that is native to the Santa Monica Mountains, although four invasive species have been introduced there, and lack a white margin on the flower. As with other typical members of the Euphorbia family, it has a white milky sap, and is poisonous. It is one of only 11 members of the Euphorbia native to California, and one of four native to the Santa Monica Mountains. Euphorbia albomarginata is a common ground cover plant, usually growing less than 1/2 in (13 mm) high, with individual plants covering about a square foot, often growing closely and forming mats of vegetation. The flowers of this plant are tiny and edged in white, with a purplish center. It can be found in open fields, on roadsides, or anywhere where the ground is disturbed, including ornamental gravels in suburban yards, where it is considered as a weed.