Plants erect, usually unbranched, not deep-seated in substrate. Roots diffuse. Stem unsegmented, green, spheric or ovoid to short cylindric, 3-20(-38) × 2-12 cm, glabrous; ribs 13-21, crests deeply notched and usually evidently or prominently divided into tubercles, with fertile portion as short adaxial prolongation confluent with spine cluster or connected to spine cluster by very broad groove, grooves obliterated or obscured by development of flowers; areolar glands absent or present in Sonoran Desert species; cortex hard or firm, cortex and pith mucilaginous. Spines 12-30 per areole, partially or entirely hiding stem surfaces; radial spines 9-26 per areole, straight or curved, 3-37 mm; central spines (0-)1-9 per areole (central spines of immature plants absent in some species), straight or curved, not hooked [or rarely so in E. unguispinus of Mexico]. Flowers diurnal, near stem apex, from adaxial ends of short areolar grooves, occupying and greatly distorting most of groove, campanulate or funnelform, 2-7.4 × 2-7.4 cm; outer tepal margins entire or undulate; inner tepals white or colorful; ovary scaly, spineless; scales 1-20(-34 in some E. johnsonii), broad, margins scarious, axils naked (or minutely hairy in E. johnsonii); stigma lobes red, brown, or green (yellowish white). Fruits indehiscent or splitting longitudinally (only circumscissile near base in E. intertextus), green, brown, or dull pink, spheric to ellipsoid-cylindric, 6-20 × 5-15 mm, scarcely succulent, quickly drying, scaly, hairless, spineless; scales 1-20(-34), broad, scarious; pulp colorless; floral remnant persistent. Seeds black or nearly so, spheric or hemispheric, 1.8-2.5 mm; testa cells strongly convex. x = 11. Two phylogenetic studies of chloroplast DNA sequenced (J. M. Porter et al. 2000; R. Nyffeler 2002) support the conclusion that Echinomastus is the genus most closely related to Sclerocactus. Athough morphologically cohesive, Echinomastus is inferred to be a paraphyletic group, with some species (i.e., E. johnsonii) more closely related to Sclerocactus than to other members of Echinomastus. Further studies will be required to determine whether those genera should be combined under a broader circumscription of Sclerocatus. Without explanation, L. D. Benson (1982) erroneously extended the name Neolloydia to this group of cacti, which is not closely related to Neolloydia.