Sources : Skate

Pliny the Elder [1st century CE] (Natural History, Book 9, 14; 9, 67): [Book 9, 14] The aquatic animals have a variety of coverings. Some are covered ... with rough skin which can be used for polishing wood and ivory, as skates... [Book 9, 67] In a similar manner the skate and the turbot while in hiding put out their fins and wave them about to look like worms... - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:37): The squatus is named because it has sharp scales [squamis acutus]. For this reason wood is polished with its skin. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Fish 7.78): According to Pliny, the scuatina is a sea fish. It is five feet in length, and has a one foot tail. Hidden in the mud, it suddenly catches the fish. The skin of its body is so rough that wood and ivory can be polished very effectively with it. But the hair on its skin is short and black, like dry grass, but much shorter. Providence has given it such a hard nature that it can scarcely be cut with pointed iron or steel.This fish signifies the ungodly, whose cruelty tries the patience of the blessed, that they may be well polished and more gloriously placed in the house of the Lord. - [Badke translation/paraphrase]