Opus tessellatum

Opus tessellatum mosaic (3rd century); the refers to the mainly vertical rows in the main background behind the animal, where tiles are not also aligned to form horizontal rows.

Opus tessellatum refers to a type of mosaic made from tesserae.

Opus tessellatum is usually used for backgrounds consisting of horizontally or vertically arranged lines — but not both in a grid, which would be "opus regulatum".