In snakes and amphibians, the canthus, canthal ridge or canthus rostralis,[1] is the angle between the flat crown of the head and the side of the head between the eye and the snout,[2] or more specifically, between the supraocular scale and the rostral scale. It is defined as a sharp ridge in many viperids, but is rounded in most rattlesnakes, for example.[3][4]