Per, shrub. ST twining (elsewhere sometimes prostrate to ± erect). LVS opposite; blade often linear to narrowly lanceolate or hastate. INFL axillary, often umbel-like. FL: corolla gen ± spreading to ± erect, with ring of tissue at base; filament-column appendages free, ± spheric, attached to base of filament column, projections hollow; pistil head ± conic, 2-lobed, or both. FR gen erect, narrowly fusiform to narrowly ovoid, with fine longitudinal grooves.
(From Jepson Manual).  Note that in S. hirtellum, the filament-column appendages are fused to the ring of tissue.

Two species in California, S. cynanchoides ssp. hartwegii and S. hirtellum. Both species in both deserts and sometimes reaching beyond the desert. Bloom period according to Munz, cynanchoides: April to July; hirtellum: March to May.

Key in Munz, A Flora of Southern California

Corolla purplish; plant glabrous or with subappressed hairs ............S. cynanchoides
Corolla greenish-yellow; plant with short spreading hairs ................S. hirtellum

Key in Abrams, Illustrated Flora of the Pacific States

Plants nearly or quite glabrous; flowers purplish; scales of the inner crown globose-ovoid, broader than long, free from the annular crown; fruiting follicle solitary, perpendicular to the pedicel ......... Funastrum heterophyllum (Sarcostemma heterophyllum, Sarcostemma cynanchoides)
Plants densely hirsutulous; flowers greenish yellow; scales of the inner crown oblong-ovoid, distinctly longer than broad, adnate at base to the outer annular crown; fruiting follicles usually in pairs, diverging at right angles to the pedicel .......... Funastrum hirtellum (Sarcostemma heterophyllum var. hirtellum, Sarcostemma hirtellum)

Additional NOTE in species descriptions:
cynanchoides: leaves short-petioled, narrowly linear to linear-lanceolate, 1.5-5 cm long, 1-10 mm broad, the broadest often subhastate at base.
hirtellum: leaves less variable than in the preceding, narrowly linear to narrowly linear-lanceolate, 1.5-4 cm long, 1-4 mm broad, acute to obtuse at base, never hastate.

Key in Jepson Manual

Pl. sparsely hairy; corolla ± purple; filament-column appendages free from ring of tissue at base of corolla ............ S. cynanchoides ssp. hartwegii
Pl. densely hairy; corolla ± greenish white; filament-column appendages fused to ring of tissue at base of corolla........ S. hirtellum

Additional NOTE in species descriptions:
cynanchoides: leaf blade base gen hastate or truncate; corolla pink to purple, or lobes white with purple streak.
hirtellum: leaf blade base variously tapered; corolla white to greenish-white.