Rafinesque, Bulletin Botanique 1: 216, 1830
(Also Opuntia mesacantha Raf ssp mesacantha Majure)
Opuntia mesacantha is a southeastern plant found in GA and AL. It resembles O. humifusa and O. cespitosa but has distinct differences; see Majure, 2014 and Majure et al, 2012. Plants are prostrate and may branch in all directions to 3-ft across. Cladodes may be spined or unspined and are smooth in outline, obovate or rotund, occasionally elliptical. Spines are stout (0.95-1.3 mm in diameter). See the original description. O. mesacantha is tetraploid, whereas O. lata is a diploid. (Majure treats the two as O. mesacantha ssp mesacantha and O. mesacantha ssp lata.) The two taxa are difficult to differentiate in the field, but Adanick et al., 2018 developed a method to measure stomates in the field and differentiate the two taxa easily.
Concerning the differences between O. mesacantha and O. humifusa, The Flora of the Southern and Mid-Atlantic States (2015) reports:
O. humifusa is an allotetraploid (2n=44), cryptic species that is most easily confused with O. mesacantha ssp. mesacantha, from which it can be separated by its lack of spines…and generally increased number of areoles per diagonal row across the cladode face at midstem (4-5 vs. 3-4 in O. mesacantha), generally inserted glochids (vs. exerted in O. mesacantha), and smaller seeds (4.0-4.6 mm long vs. 5.0-5.9 mm long in O. mesacantha ssp. mesacantha) with a smooth funicular envelop (instead of the upraised funicular envelope in O. mesacantha ssp. mesacantha).