The genus consists of sea urchins with rounded (but slightly elliptical) test, with the peristome (mouth) located in the center of the oral surface (lower) and the periproct at opposite, at the apex of the aboral (upper) face.

The ambulacrum are polygeminate, with between 9 and 14 pairs of pores per plate, arranged in more or less regular arcs, sometimes almost in bands.

The interambulacral plates bear a single, massive nipple, flanked by secondary tubercles on the aboral surface.

They are never modified into scales or plates (unlike Colobocentrotus), but the secondary, short radioles can have an almost rounded shape This genus contains two species:[2] These two species are very close visually, and often difficult to distinguish, since they generally share the same distribution area.

H. trigonarius is also usually darker (brown, dark orange...), but the great variability of colorations in this genus makes this criterion delicate.