Selon M. Anderson, et je n'essayerai pas traduire.
"H. triangularis....Stems sharply three-angled, marginally straight or only slightly elevated at the areoles, not horny, 3-4cm wide. Spines 6-8 needle-like with swollen bases, short, spreading."
Et pour H. undatus,
"Stems jointed, stout three-winged, marginally wavy, sinuate, horny, 4-7.5 cm wide. Spines 1-3, short, conical, brownish gray to 3 mm long"
Ca fait, si les cotes sont ondulantes, c'est plutot H. undatus.