Note how the shape is roughly opposite to that of a PRC to light pulses, i.e., at phases at which light elicits phase-delays, melatonin produces advances and vice versa:
The lizard PRC was actually constructed in our lab, about ten years before I joined. The article, though, gives the wrong reference to this:
Underwood, H. and M. Harless (1985). “Entrainment of the circadian activity rhythm of a lizard to melatonin injections.” Physiology & Behavior 35(2): 267-70.
In that paper, lizards were entrained by daily melatonin injections. The PRC was reported in a different paper the following year:
H Underwood (1986) Circadian Rhythms in Lizards: Phase Response Curve for Melatonin, Journal of Pineal Research 3 (2), 187-196.
Update: an alert reader sends a better figure, taken from this freely available recent paper: