In snakes, the canthals are the scales along the upper surface of the canthus rostralis

Canthus (snake)

In snakes and amphibians, the canthus, canthal ridge or canthus rostralis, is the angle between the flat crown of the head and the side of the head between the eye and the snout. More specifically, between the supraocular scale and the rostral scale. It is defined as a sharp ridge in many viperids,...

. They are located behind the level of the prenasal/postnasal suture and before the supraocular. They are also described as being located along the edge of the crown between the internasals and the supraoculars. If these are large scales that contact each other along the midline, they are more properly referred to as prefrontals.