Lacunal scales are those scales

Snake scales

Snakes, like other reptiles, have a skin covered in scales. Snakes are entirely covered with scales or scutes of various shapes and sizes. Scales protect the body of the snake, aid it in locomotion, allow moisture to be retained within, alter the surface characteristics such as roughness to aid in...

 that form the inner border of the loreal pit

Loreal pit

The Loreal pit is the deep depression, or fossa, in the loreal area on either side of the head in crotaline snakes . It is located behind the nostril and in front of the eye, but below the line that runs between the centers of each. It is the external opening to an extremely sensitive infrared...

 in crotaline snakes

Crotalinae

The Crotalinae, commonly known as "pit vipers" or crotaline snakes, are a subfamily of venomous vipers found in Asia and the Americas. They are distinguished by the presence of a heat-sensing pit organ located between the eye and the nostril on either side of the head...

. Usually, these scales are large and also curve outward to form the outer border of the pit. In most cases the pit opening is triangular with one apex pointing back towards the eye. It is bordered above, below and in front by one or more supralacunals, sublacunals and prelacunals respectively.