Their specific epithet is from the Latin word modestum, meaning modest or calm.

Most are uniformly grey in color, but they can also be light brown, or even a pale yellow.

Often, darker-colored regions occur around the neck and groin, and sometimes striping on the tail.

Unlike other horned lizards, they lack a fringe of lateral scales and do not seek to flatten themselves to the ground to eliminate shadow; instead, they hunch their bodies into the shape of a rock for camouflage, boldly casting a shadow.

[4] Roundtail horned lizards are oviparous, breeding and laying eggs in early summer.